Damn, wish I had it explained that clearly when I tried it. One note to add to Ree's assessment. If you're going to a 27 tooth sprocket, then you'll have to grind down one of the reinforcing ribs in the cover. It only needs to come down a little, but the chain will rub against it if you don't.
I've heard that you cant go any smaller with the back sprocket (there just isn't any room), and I've never seen a larger rear sprocket. I've wondered if a wheel from a different ped might work in the back ??