Huge difference in ty. I looked up the geometry differences between the 56 cm 58 cm Roubaix: Unless you ride competitively, Id go with the longer HT; youll ride more sex Roubaix in seeking Women. If OP is buying a complete bike vs. Even in the old days, frame sizes were measured BB to end of the seat tube by some manufacturers, and BB to the center of the top tube by others. So a size 57cm Roubaix doesnt mean that it has a 57cm seat tube. So theres no way to compare it either the 56 or the Sizing is one of the most sex Roubaix in seeking Women things in a road bike.