So my group is really tiny (3 players including myself) so I am allowing them to supplement their numbes by useing soldiers we hire or meet along the way, servitors and such. They are searching for a Inquisitor who has accused them of heacy (they decided to call for the inquisition on a small world that had a large demon presence) and they tracked him to a hive world that is beseiged by chaos and the imperial guard. I have read up on the legio titanicus and woud like to have every few rounds they are forced to hide as a stry volcano cannon round levels a nearby building, or the street is filled with bolter rounds, but how big of a titan would be sent to a hive? Further would it even be fair to ask them to try and kill it?

I would avoid letting a RT group face off against a titan. In fact, I would incourage them to run… If you have the Deathwatch: Rites of Battle, it has rules and stats for the Warhound Titan. Between the fearsome armament and massive armor, Titans have void shields.

"Void shields count as a barrier with 25 AP that surrounds a Titan in all directions at a 5 meter distance. To disable a void shield, attacks must deal 20 damage past the 25 AP. This damge is cumulative,to represent the void shield slowly overloading, but a weapon's penetration has no effect on the void shield. Attacks from within the 5 meter protection distance ignore the void shields and directly affect the Titan.

The Warhound has two void shields, each of which must be bought down before any ranged attacks outside the void shield radius can affect the Titan. Once a shield is brought dowm the Tech-Priest can attempt to restore them by making a Difficult (-10) Tech Use Test. Success means one shield is restored in 1d5+5 rounds. This time is reduced by one round for every degree of sucess, to a minimum of one round."

If you are still interested in using a Titan, let me know and I will get you the rules…

It depends on the size of the war. Local conflicts are unlikely to warrant sending even one Warhound scout titan, let alone a battle titan, unless it was virtually in-system when it broke out and got short-stopped, or there was someone/thing of serious import to the Collegia Titanica or the Adeptus Mechanicus in general (or to someone with pull with one of the aforementioned bodies) that was imperilled by the war.
Hell, if a local war (that didn't meet the extraordinary parameters I mentioned) warranted any AdMech/titanica involvement beyond the usual support of Munitorum forces (the Guard and Navy) a local war might get sent Skitarii, and maybe a Knight* Household if they hadn't generally been consigned to fort guard duties on forge worlds.

In the normal course of wars, however, the scale and general importance of a conflict is the determining factor for what size of titan (if any) is deployed to a world, with escalating levels of combat determining the mandated level of response, not the terrain they are likely to be fighting in. The only real limiting factor in terms of terrain is the kind of hive and how much collateral damage is acceptable- a fully sealed underwater or hostile environment hive with an intact pressure hull is only going to fit titans up to a certain size, and will likely preclude the use of certain weapon systems (like Volcano Cannon). Anywhere else, the majority of buildings and internal structures of a hive are basically just so much soft cover, so if there's a reason to deploy a titan there, it can maneuver and fight. There may not be much left in that area afterwards, but they can maneuver and fight.

However, as I said there must be a reason to deploy them, and it boils down to "someone who matters thinks this fight is important enough to call in a stick this big."
As a general rule, that makes the war pretty **** important to have the engines of a Titan Legion to Walk upon a world, but it is entirely possible that they were simply the closest available force (aside from PDF, SDF and Arbites) and got pulled to act as fire brigade, or the Legion's collegiate council decided that they were going to Walk to the rescue for their own reasons (a good back-up escuse, as they have a very short list of people they have to tell why- maybe the Master of a Forge World, an Inquisitor or a Warmaster/Lord Commander Militant; probably the Master of their home Forge World and members of the High Lords and Senate other than the Fabricator General; and definitely the Fabricator General).

Having said all that- I personally wouldn't deploy anything larger than a Warhound, or at most a Reaver into any hive, not if I wanted there to be a functioning hive afterwards. If I was indifferent to the hive's survival, or desperate, I'd send in battle titans like Reavers and Warlords as I had them. If I wanted the hive gone (either shattered and rebuilding for decades or wholly destroyed), or was at the point of having to raze it to deny it to the enemy, I'd call in the big boys: the Emperor titans (or lots of smaller with orders to wreck the place rather than relying on collateral damage).

As for killing a titan? The best way is to nuke it from orbit. It's not the only way to be sure, but it is the best way. The problem with that method, of course, is that orbital bombardment is not the most precise of weapons, so if there's something or someone you want to keep alive nearby, they may well be in serious trouble (and, of course, you need to have a ship in orbit ready to do so, and either comms with the ship or actually be on it).
Next, there's the usual array of warfare options (other titans; lots of artillery; way more aeronautica; a couple of tank companies, preferably dedicated tank hunters, or even better: super-heavy titan-hunters like a Shadowsword). You basically need a big and tough enough target to batter down the titan's void shields and then either knock it over, inflict enough damage to mission-kill its weapons, or breach the head or reactor. While surviving the fire it's going to be shooting back at you. The best way to do that is to either hit it from beyond line-of-sight and hope it doesn't have any indirect fire weapons, or try and ambush it from close range (ideally inside the void shield perimeter). If you can't manage that, it's down to hit it with lots.
On the infantry level, in some ways it gets easier to take down a titan. In most ways, however, it gets a lot harder: Infantry are hardly even crunchies to titans, more like squishies, and that's without being hit by titan grade weapons. 90% of titan grade weapons won't leave enough of a human corpse to identify without DNA analysis, and generally will take out a whole squad at a time. In addition, the majority of man-portable weapons will barely scratch a titan's armour. That's not to say it's not possible: anti-tank weaponry, focussed on the knee or ankle joints can actually bring one down**, assuming you don't have to worry about the shields.
And that is where the infantry comes into their own. There are virtually no situations in which an infantryman will carry enough kinetic energy to be stopped by a titan's shields, and with a few special exceptions (such as the bastions built into the lower leg of Emperor titans), titans don't have much in the way of close-range antipersonnel defences. So, if they can get close enough, a couple of guys with, say, plenty of melta bombs can kill a battle titan, and only really have to worry about kicking/trampling, and the titan's support troops (and possibly the crew).
Of course, the trick is getting close enough. A titan's auspex systems are really rather impressive, so it's a case of either trying to sneak up while it's otherwise distracted (and risk getting hit and killed by whatever is doing the distracting), using buildings and trickery to lay an ambush (and pray the princeps, moderati or the titan's machine spirit don't think the faint thermals or whatever look dangerous), or ideally sneaking up on it while it's standing down for rest and maintenance (yup, it does happen in the field. Occasionally. Don't count on it though).
In all those situations, a small group has a better chance of getting in close than a massed formation of troops. The trade-off is they may not have the firepower to kill it, and a smaller group runs a proportionally higher risk of everyone being killed when the titan notices them and decides they're a threat.

In the case of larger titans, it's also possible to take them out of the picture by boarding them, but that's a really ridiculously risky (and awesome) tactic. It requires someone to find and climb to an access hatch (rarely below knee height on the titan, usually at least hip/crotch), which will be sealed during battles, somehow open it (setting off internal alarms), and then fight fanatical crewmembers, many of whom will be carrying rather impressive anti-personnel weapons, and all of whom will know the terrain better, and try and make it to somewhere sufficiently important you might be able to talk them into surrendering (or somewhere a good explosion will kill the titan and everyone aboard). While the titan is moving, and while avoiding being killed by internal mechanisms you probably know nothing about.
Try it on something like a Warhound, and you won't get anywhere, as they're essentially too small for proper internal spaces (and the only access hatches are on the upper surface, iirc). Try it on an Emperor and while they do have access hatches below the knees, they also have lower legs that incorporate fortress bastions with plenty of close-range heavy bolter emplacements, designed to be filled with a company of skitarii tech-guard. Each. Plus often another company in the fortress bastion on their shoulders, and the usual complement of security troops and armed crewmen.

So… yeah. It is possible for a small group of people, on foot, to kill a titan. It's just not very easy, and unless they're incredibly smart and lucky, not particularly survivable. If you're interested, the canon record for the smallest group of people to kill a titan on foot is something like 4- an Iron Warriors chaos lord and his personal retinue, while the record for boarding and hijacking one is either 5 (a squad of Imperial Fist scouts, on a renegade Reaver titan. Caught it stood down for maintenance, killed everyone aboard, ate their brains and used their borrowed skills and the manual back-up controls) or 3 (a space marine captain- coincidentally once a member of the aforementioned squad, a Squat engineer and a renegade Inquisitor, posing as a Commissar; tricked the princeps of a Warlord into giving them a lift then pulled guns once they were up by the control room) depending on whether or not you count taking a "friendly" titan as a proper hijacking. The all time record for killing a titan, however, is probably Wazzdakka Gutzmek, who drove his warbike off a cliff into the head of a titan. While on fire from passing through the shield.

* It's a pity, really. I like the Knights as a concept. Think one man mini-titan, or something like a 'mech from the Battletech setting. The only issue is that a) they were pretty much all withdrawn to act as garrison troops during the Great Crusade and b) the biggest weapon they mount is a standard battle cannon, just like that mounted on a Leman Russ, so no Volcano Cannon as a random event if you use them.

** Titanfall is apparently one of the most awe-inspiring sights in a battle. And it has the advantage of effectively killing the titan, even though it may still have operational weapons systems, and/or angry surviving crew, as even on low-g worlds like Mars, once a titan falls over, it can't get back up. Not without a serious set of cranes to lift it.

Can they call in a Macrocannon barrage?

If no, than don't ask them to try and kill the Titan.

Thanks for all the feedback, as it turns out the did exactly what you guys sad they should do… they bombed the **** out of it from orbit (thry rolled charm so well the Imperial Navy joined in) and so my great titan chase was kinda anti climatic…

That's a real shame. I'd have had the titan near enough to the party to make orbital bombardment difficult. I like the idea of the party calling in an orbital strike and being politely informed they are going to be in the blast radius. If I was to have a Titan in my campaign I'd run it as a challenge that the party have to pass a series of skill tests in order to overcome or have the Titan as a prize for completing an endevour (such as locating a lost Adepts Mechanicus fleet).

Kahadras

IMHO, it would be a very bad idea to give players a Titan =P

For one, it goes completely, absolutely, unargueably against fluff. The only possible way I ever see players obtaining a Titan is finding one that's been somehow inactive for a long, long time. But then there's the problem of running it. It is no simple thing, the running of a Titan. There are hundreds, if not more, of rituals that the Mechanicus Princeps, Moderatis and Tech-Priest must perform just to operate the machine god (going on fluff).

That, and a Titan is the most powerful vehicle in the entire game, including all 5 tabletop series (DH/RT/DW/BC/OW) and excluding starships.

The larger Titans can actually fire at and destroy void ships. This means that Titans are the equivelant of a planet-based void ship.

This being said, my absolute dream is to play a game similar to Rogue Trader, but instead called Legio Titanica or something similiar, where all the players are the crew of the Titan, which would have a bunch of similarities (equipment and component-wise) to a void ship.

I once gave my players a Reaver titan that needed a ton of work and repairs… There were eight people playing at the time. The ork, tech priest, and techmarine got together and SCIENCE happened. After the titan was repaired they set about starting a crusade for fun and profit and then charged off at a eldar craftworld. Three months later everyone was dead and epic drinking stories were told. My suggestion. Give them a titan and watch as the neighborhood blows apart around them.

Whats the big deal no one here ever played Mekton or Gundam RPGs??

Make some knight Titans go to town with the idea. obviously if you’re gonna dance the “Titan dance” then fight other titans, chaos Knight Titans, killer Kans, ork Gargants, Big Meks, Stompas, really large Squiggoths, gaint tanks and crap like that, it makes for some awesome games watch some MS Igloo 1 and 2, Mobile Suit Gundam: The 08th MS Team, and my favorite Metal Skin Panic MADOX-01 and see what a pain in the a$$ it is to field those things obviously your players can’t field them everywhere. You just use common sense, if an adversary knows that you have one or two then he’ll be ready for them with haywire grenades and missiles, and magnetic mines you blow a leg off that thing isn’t going anywhere. There’s a lot of great material on the web about giant mecha, the movement of the Knight Titans are supposed to be fast and fluid but they suffer from being really huge meaning they have limitted battlefield use. As far as rules well take a warhound and shave 25 – 35% off (or more) all the stats and there’s your Knight Titan (or I’m sure somewhere someone has done some stats for it in "RT" rpg forum).

Good luck and have lots of fun with it (remember you’re the only one that makes shyt cannon in your game).