Hi Joy, you mentioned that you thought a shorter seat tube implied a shorter to tube. Sort of. You'll find a lot of bikes these days have sloping top tubes and define both the top tube and seat tube as 'effective' length. In many cases manufacturers are only making small, medium, and large frames. The 'cockpit' fitting comes by fitting the correct stem and seatpost. Your best bet would be to get fitted at a reputable shop. If that isn't practical, there are on-line tools where you enter your measurements like inseam, torso length, and arm length, and it will suggest a frame size and stem. From there you would have to do some additional math is your choosing a non traditional frame design. If you let us know if some specifics, I'm sure there is enough expertise here to guide you in the right direction