Something many cyclists have difficult time understanding is frame size. Because there is not standard for frame geometry or for measuring a bike or a person a cyclist can ride 2, 3, or 4 different sizes depending how the frame is contructed and measured.

In reality you should be measuring how high the top tube is off the ground (standover height) versus the length of the seattube.

Standover height takes into account the differences in bottom bracket height and measuring methods. If you measure your inseam length your standover height should be less than that. The numerical value assigned the frame is is irrelevant.

Example: 3 bikes, all with 75cm standover heights.

Bike 1 has a BB height of 25 cm and is measured C to C.
Bike 2 has a BB height of 24 cm and is measured C to Top.
Bike 3 has a BB height of 26 cm and has a sloping top tube.

If bike 1 is a 54cm, bike 2 would be a 56cm and bike 3 would be a 52? So, which will it be? They all have the same standover heights....the one thats most comfortable.