I was at a church gathering recently and there was a 10-ish year old kid who couldn't stop talking about MineCraft. He was really impressed when my wife told him she'd written a mod for the game (the original Marble mod with the texture that tiled over multiple blocks).
Assuming I ever finish, it'll have a central chest-high "lava blade" as the wiki called it to kill monsters. The rest is just a large area with water flows to push monsters towards the water blade.
It's big enough that cobblestone generator is pretty much mandatory. It probably spans roughly an 16x16x8 volume, though not all of that is filled in for obvious reasons.
Is your monster harvester similar to http://ripminecraft.blogspot.com/2011/08/monster-condo.html ? Larger? Smaller?