Depends on the mgf. A 55cm in one bike brand can be different from a different brand 55cm. All the geometries have a roll in different frame sizes. The size listed is the seat tube measurement. This will vary from maker to maker as some measure center of bottom bracket to center to top tube, and others measure center of bottom bracket to top to seat tube. This will usually be specified in the makers geometries charts - some times not and you will need to contact the maker to find out. However, top tube length is really the most important measurement when choosing a frame size. And, a 55cm frame in one maker can have a 55cm top tube and another maker can have a 56cm top tube. The geometry charts really need to be paid attention to.