I run 25s on both bikes. The play bike is just a 650 with average head, cams, and piston, and I run either a 13 or a 14 paddle extreme, 81 rollout. Roughly a 60 horse bike. Since tires are so damn expensive now, I would recommend buying a set that would be considered to much tire and gear accordingly. That way you can grow into them. 22s are cheaper, and will work ok, but the DS likes the 25s so much better. Especially once you start making good power.