The seat tube and top tube lengths are two of the three main measurements required for bicycle sizing. The third is the seat tube angle, which is usually influenced by your body size. The length of your legs and arms is then used to arrive at a seating position that is most comfortable.
Generally, the horizontal distance between the centerlines of head and seat tubes is taken as tube length whilst seat tube length is the straight-line distance between the centers of the bottom bracket and junction between the seat and top tubes. The seat tube length needs to be two-thirds of your inside leg measurement which is taken in an upright standing position without shoes and with your feet placed 10 inches apart. It is the vertical measurement from the floor to the crotch – the lowest part of the pelvic bone.
The top tube length of your bicycle is proportional to both your trunk and the length of the arms. Trunk length is measured from the seat to the top of your shoulder. The forearm length is measured from the back of the elbow, which is pressed against a vertical support such as wall, to the center of your clenched fist.
These measurements are taken in a seated position with your back firmly pressed against a vertical surface (wall). The arm length is obtained, with your arm stretched out straight in front, by measuring from the vertical support (wall), to the center of your clenched fist. These measurements are then used to calculate the tube length using a standard formula.
For the seat tube angle, you need measurements of your thigh and lower leg lengths. The thigh length is measured horizontally from the vertical support to the front of your kneecap whilst the lower leg length is measured vertically from the top of your kneecap to the ground.
It is important to remember that no formula can give the perfect frame size and riding position for every rider. Your riding position can be finely tuned after you have arrived at some comfortable positions after several rides. The seat angle for tall riders who generally have longer legs is around 75 degrees. Since these riders can sometimes face backache problems they need to adjust the seat angle so as to reduce the possibility of back problems.
The bicycle sizing should normally be done by the fitters employed by the bicycle shop owner. They can take accurate measurements and fix the bicycle as per your needs.