Beast's Castle is definitely the smallest (besides Atlantica, and possibly the Hundred Acre Wood). >_> Not much to it.
Also, I agree it isn't fair to lump Hollow Bastion/Radiant Garden and the Hundred Acre Wood and Space Paranoids all together like that. I mean, they may be worlds within worlds, but they're still intended to be considered worlds. The Coliseum and the Underworld, the Disney Castle and the Timeless River, and the two holiday Towns are okay because they're intended to be two parts of the same world.
And I'm pretty sure Pride Lands is the biggest. xD