I normally ride a 49-51cm seat tube but after looking at the specs, the 50cm really has a 46cm seat tube c-c, so I was considering the 53cm size. But, I ride a 52cm top tube with 9cm stem on all my bikes so I went with the 50cm size since it has a 52cm top tube (and the $125 January special help the decision too).

It is a perfect fit. So simply put, you really need to know what top tube you ride.