Summon Swarm questions

[Nareau]

PHB, 261
Effect: Swarm of small creatures in a 5-ft. spread.

Does that mean it fills four 5-ft. squares? A spread is centered on the junction between squares, so it seems like it should. But some "spread" spells specify a radius. Take fireball, for example:
Area: 20-ft.-radius spread

Does the difference lie in "effect" vs. "area"?

Secondly, it says that you can't cast spells while you're in it. So if I cast this on an enemy wizard, can't he just take a 5-ft. step out of it and cast his spell? Granted, he'll take damage for that. But since he's out of the effect while he's casting, I don't see any reason he should even have to make a concentration check.

 

[TrizzlWizzl]

p. 69 DMG, under 'Bursts and Emanations':To employ the spell using a grid, the caster needs to designate an intersection of lines on the grid as the center of the effect.

So a summon swarm would fill a two square by two square area... just like you assumed already, pretty much. With regards to 'burst' or 'spread' or whatever, I think that informs the manner in which the effect is able to move around corners, fill up the area, etc.

And yeah, I guess a caster would be able to just step out of the area of effect (suffering damage).