6' tall myself and I ride a 55cm kilo which has a 56 top tube and my road bike is a 54cm with a 55cm top tube. Fit has nothing to do with how much clearance you have over the top tube, sure I have more room on the 54 but both bikes fit the same after I fit them to my dimensions. The only time I would worry about more top tube clearance would be on any off road bike, on a track bike on the street it's not really a big concern.