How do you resolve an attack with Blast against a large monster? I'd guess you choose a space in the target monster's square that you can see and hit each other monster adjacent to that square.

That is correct: You choose a square to attack which happens to have a monster in it, and you measure blast radius from that space.

Just to be clear (and you probably know this), you have to specify your target space BEFORE you roll. You can't roll. see what square would be the best one to pick for major blast damage and pick it.

Ok, in that case we were playing it correctly. Thanks for clearing it up!

And just to be clearer: You select one of the monster's square to determine LOS. If you have line of sight, you can attack, and if you roll enough range, you hit. If you use a blast ability, ALL figures adjacent to that MONSTER (not square) also get hit by the attack. This is why attacking large monsters with blast is extremely beneficial. More bang for your buck, so to speak.

If you use a blast ability, ALL figures adjacent to that MONSTER (not square) also get hit by the attack. This is why attacking large monsters with blast is extremely beneficial. More bang for your buck, so to speak.

I'm sorry, but as I understand it, that's exactly wrong.

Blast reads "This attack affects all figures adjacent to the target space." Target space, not target figure. The large monster is not affected by the attack twice (even though it has spaces adjacent to its target space,) but all other adjacent spaces get affected. In this manner, blast is actually less effective against large monsters.

This is consistent with the general mechanic in Descent where attacks target a specific space that an enemy figure is in, they do not target the entire enemy figure.

Zaltyre is definitely correct. Like any other attack, you are targeting a SPACE, not a figure. If you hit with Blast, any figure adjacent to the targeted space is affected.

Base Rules, Page 13:

Blast
On a Blast attack, all spaces adjacent to the targeted space are also affected
by the attack. The attacker rolls his attack pool once and applies the
result to each affected figure, but each affected figure individually rolls its
defense pool separately. Only the target space needs to be in range and in
line of sight of the attacker. Blast attacks affect both enemy and friendly
figures, and a figure can only be affected by Blast once per attack.

same here. It's the "square", and a same monster cannot be hitten twice by the same attack because he occupies multiples squares

(seen it somewhere on bgg faq)