I've read lots on how to size a 'cross bike based on what your road bike is like...but I don't own a road bike! Any tips on going from a mountain bike to a cross bike? I ride a 15.5" mtb frame (I'm about 5'6"), with a pretty long top tube...not really sure what that translates into in terms of a cross bike. I looked at a 52cm frame recently, which seemed to fit ok in terms of length, etc... but I had zero standover. Should I error on the small end...especially if I intend to race?