Originally Posted by Bob the Mech

The bigger the frame the longer it gets. Inside leg sizing only factors for standand over height and saddle to pedal distance. Reach factors for torso and arm distance. Stack factors for how low or high the front end of the bike is. Low stack means a short head tube and a more agressive riding postion in road bike terms, puting the rider in a lower flater postion. a higher stack means a longer head tube, puting the rider in a more up right postion.

If you are going down the used bike route and your not armed with a list of measurements then it's doing it the old fasioned way...Get on the bike see how it feels. It's probably the case of trying a few before you find the right fit.