As for roster size, it really depends. If you've got 10 committed kids who are durable, that will work fine, and everyone will probably be pretty happy. If the kids have trouble making tournaments and/or are fragile, 12 might not be enough. People worry about game injuries while playing with only nine, and it's certainly possible, but in 11 years now, I've never been part of a team that had a problem while doing that.