Originally Posted by
Rolla
I'm 5' 9" with a 33" inseam. I ride a 54cm road bike and a 54cm gravel bike. That's "my size," just as it has been since about 1979. But on an All City Nature Boy, I'm a 49cm. On a Breezer Radar, I fit a 51cm. On a Black Mountain Road Plus, it's a 47cm. In general, the top tube length, stack, and reach are the measurements you need to pay attention to; these days, you can pretty much ignore the seat tube measurement, and any S, M, L, XL designations.
I agree with the above, except when it comes to Canyon. Take a look at their size designations and the geometry associated with those. It's totally silly.