What Mountain Bike Size Should I Get?

infernolism asked: Okay, so i’m a 15 year old dude thats 5′9″. Do you think i can use a small bike like that size of an average bmx bike, but with gears? I just want a small bike but with gears.

single track mountain biking with a bmx bike?

Rebecca M asked: Do you think it is a bad idea to single track mountain bike with a professional bmx “race” bike. It seem to be physically punishing. the last time i rode my back got all jacked up. what kind of safty gear?