Clearly not. The quotes you selected are not contradictory. A "blast" must satisfy both. The blast must be adjacent to the creature. Therefore the Large or greater creature must choose an origin square in its space which permits that.Can a Large creature blast itself?
pg 272 PBH
a blast fills a area adjacent to you ..
later " the blast is adjacent to the origin square which must be a square in your space..."
Another 'why' scenario:
I don't have the rules with me, but I recall reading that you can enter a square of a creature 2 sizes larger than you--perhaps a dragon wants to scorch a halfling rogue that just rolled underneath him and bloodied him.
I don't think they can but there's a lot of reason why they might want to.Yes, they can. But why would someone do it?
right. From a RAW standpoint it looks as if this is not allowed but it's pretty easy to see how a dragon might do this by arching his neck and firing downward.a flanked dragon, which probably has resistance to its own breath, has pretty good motivation.
It just happened in one of the PbP games...
some at wills can immobilize or stop movement and dnd is an exception based rules system so there's always a possibility that someone will be stopped in the square. At which point the creature might want to target you with the blast.Well, that is a rather interesting point. Hadn't really considered that. You are correct, there is something about 2 size level differences. PHB 283, you can move THROUGH the space of a creature 2 sizes bigger/smaller than you, but you can never end movement in another creature's space, unless it is helpless or one or the other creature is tiny.
So this scenario is effectively irrelevant because the only attack that might happen while a PC is IN a monster's space would be an OA. I don't know of any creature which has a blast power that can be used as an OA. Were there such a creature, then it might potentially have a reason to want to do this.
there's also the 10th level rogue utility power "close quarters". that one allows the rogue to "ride" the enemy, and stay in his space.
Also, if you allow a dragon to do this, why can't a Wizard use Thunderwave from the back edge of his square and clear away 5 of the squares adjacent to him?
Well, the game is only a simulation up to a point - you can't model everything, so some divergence is worth it. In case of the squares, that's just simplification, you're deciding that it's not worth tracking non-square regions for simplicity. For gelatinous cubes; likewise (and note you can re-fluff). It's simpler to have one rule; so you avoid, where at all possible, making special rules.I'd be careful about changing blast rules based on physics. If 4e followed physics, blasts and bursts wouldn't be squares, euclidean geometry would apply, and gelatinous cubes couldn't be knocked prone.
Also, if you allow a dragon to do this, why can't a Wizard use Thunderwave from the back edge of his square and clear away 5 of the squares adjacent to him?