WTB: Chanp Kart

Im looking to purchase a good champ kart for my son. Hes almost 12 and big for his age and I'm not sure what's best for him. I see Sr and Jr champ karts and I don't really know what the difference is. If its just that the chassis is bigger on the senior I'm sure that would work for us. Let me know whats out there please. Thanks!