In light of today's events, I thought about what size of an area a sleep spell affects and have decided on the following:
A sleep spell has an area of effect of a 15' radius from the point or person upon which it is cast (a 30' diameter in total--enough to affect a large dungeon room). As per the rules, a sleep spell affects 2d8 HD worth of creatures starting with the lowest HD monsters first.
I think this is both fair and in the spirit of other such magics with a 15' radius. Unless someone has a real problem with this, let it be written and let it be done.