After reading the entry for Shape Flesh I'm a little puzzled. I'm still a pretty new Dark Heresy player, but my Psyker's about to hit rank 4. As I was going to go into Biomancy I'm a little curious about how far Shape Flesh can be taken.

There don't seem to be a whole lot of limitations to it, in particular the description of the ability and the bit that states the Psyker can take the appearance of a creature.

In particular does this apply to size? If my Psyker, for example, wanted to infiltrate a structure and thus tried to take the form of an owl (or some form of avian creature), would his size naturally adjust or would he literally become a human-sized owl? Similarly, if, for combat, he wanted to become something much larger than a human for intimidation or the like (let's say a Grizzly Bear), would he end up transforming into just a very small bear?

I'm not the kind of person who likes powergaming, but at the same time, I do like to know how far I can take an ability.

Or is this sort of thing up to the GM's discretion?

This power is very vague. I don't believe it's intended to give the user the benefits of taking on a form other than appearing that way, so flight is out. In my campaign, we ruled the psyker could take on any shape but could not change his mass. This lead to him snaking through the vents of the facility to escape our pursuers like some kind of disgusting, fleshy worman.

Ultimately though the power is vague enough that you'll need to come to a consensus with your group as to what it can and can't do.

This power is very vague. I don't believe it's intended to give the user the benefits of taking on a form other than appearing that way, so flight is out. In my campaign, we ruled the psyker could take on any shape but could not change his mass. This lead to him snaking through the vents of the facility to escape our pursuers like some kind of disgusting, fleshy worman.

Ultimately though the power is vague enough that you'll need to come to a consensus with your group as to what it can and can't do.

Flight itself doesn't seem to be out (one of the talents/traits it can confer is Flyer or w/e the rule was). but I see. That's unfortunate, but fair, I suppose-- a rarity for this career.

Take heart in the fact that as a psyker you can always just phone up a daemon and sell it someone's soul in exchange for anything you can imagine.