I just purchased a Moab 2 Aluminum, Used. I paid about 350 for it, it looks brand new. I am about 5'9" and 155 pounds, this bike is a 15" bike. It looks a bit smaller than the other moutain bikes I have seen, I am an old BMXer so the size of this bike seems about right to me; it fits me. What is the advantage of the larger bikes, 17" 21" etc...? Is there a general rule for what size of bike a person should ride?