Does a creature that takes up multiple squares (Large or larger) have to be entirely in the area of effect to be affected by an area spell, or is it sufficient that only a single corner of his space be in the area?
It seems odd that a Colossal creature (taking up 25 squares) would take just as much damage from a fireball that includes 1/25th of his space as a human does when standing a ground zero.
The answer to your question is, yes, only a single square of the monsters space must be in it.
The reasoning for me has always been, hp doesnt really just represent injury. It represents a 'wearing down' effect. To me the hp damage from say a fireball, is really the effort expended from avoiding it, you are not actually burned by it, as that would disable just about any being. And i imagine you would duck just the same way if just your leg was about to be set ablaze as you would if the flames were headed for your chest.
It also would create massive problems in the terms of how to handle non damage spells. If i get just my big toe stuck in a web spell(firmly mind you) i am just as stuck in place as i would be if I my whole body was hooked to the web (perhaps more so as its tough to pull a weaker portion of your body free without injuring yourself).
Not to mention what about the reverse of that. If having only part of a monster in a fireball does less damage, what about having more? Why should a human who occupies only one square of a fire ball take as much damage as a colossal creature whose full 25 squares are all being burned by fire? The are certainly taking 'more' of the fireball arent they?
I think the system handles it the way it does for simplicity's sake most of all.
Think of it this way: a fireball doing 30 points of damage does it equally to everything it effects. The bigger/badder the creature or character is the more of that type of damage they can absorb. A big dragon is hurt but not as much as a commoner who would be toasted. Feats and Saveing throws allow all creatures to avoid some or all of the damage. I agree for simplicity's sake the rules cover it pretty good. The more you try to apply specific senarios to the rules the more rules you have to apply and that take away from the game play.