Mobs will spawn anywhere that fits the space (and requirements) needed to spawn. Humanoid mobs need 1X1X2 space, spiders need 3X3X2 space (but can spawn on a single block). However when you're talking about a mob trap, 20X20 is considered to be on the small side as footprints go.
For all mobs except for spiders.
For spiders
Hardware guy "Its a software problem"
Person trying to play Minecraft "THAN WTF IS THE PROBLEM?"
Playing Minecraft with a Xbox 360 controller
http://www.youtube.com/user/hydrolisk1792
by c0yote
I tried it with terrible results. I gave my wife my glasses for a second, a creeper showed up and now my wife is pregnant.
Stupid 3D..