the 13" measurement you are looking at is the length of the seat tube, which doesn't matter so much on that type of bike, sizing wise you want to be looking at TT length. you arent really going to be sitting on one of these bikes so low and out of the way is what you are looking for. my dk rig has a 22" top tube and i am 5'8" but i have long arms, for reference. I would recommend for most manufacturers you are looking for a small or medium frame, depending on your preference.