Different frames are sized very differently. What one company calls a 58cm frame could very well be smaller than what another company calls a 54cm frame. You need to know how the frame is sized before any comparison can be made.

You also need to know other dimensions of the frame. Possibly the most important dimension of the frame is the top tube length, or virtual top tube length. The actual top tube length is normally measured along the top tube from the center of the seat tube to the center of the head tube. If the top tube slopes, then the virtual top tube length is more relevant. This is the distance, measured level to the ground, not along the actual top tube, from the center of the seat tube to the center of the head tube.

Frame sizes could be measured from the center of the bottom bracket to the center of the top tube, to the top of the top tube, or to the top of the seat tube. Since the seat tube may end at the top of the top tube or extend 5 or more cm above the top tube, the same frame may be measured several different ways. Just to confuse even more, there are some frames that have sloping top tubes, that are measured to the virtual top tube, which could be even longer by several cm. In this case, what is called a 58cm frame may have a seat tube that in actuality measures 55 or less.

Most companies tell on their web sites what the dimensions of their frames are and how they are measured. You as the buyer really need to spend some time comparing these numbers to get an idea of how a new frame will compare to what you have.