I've been looking at the Trek FX 7.5, but don't know which size frame to get. I went to the LBS last week and got on a 17.5" and a 20". I didn't ride either since it was raining that day. The salesman said it looked like I was in between the two. I really felt more comfortable on the 17.5". I am 5'11" and leg length from crotch to floor is 30".