how do you determine the size of a bicycle?

Answer 1

Children's bikes are measured by wheel diameter. There are 12, 16, 20, and even 24 inch sizes. Adult bikes are measured by frame size since wheel size is generally standard (26 inch for mtb and 27 or 700c for road). Most companies measure from the bottom bracket shell to the center or top of the frame.

Answer 2

for road bikes, it is the length of the seat tube, between the clamp and the bottom bracket, measured in cm
for MTB, same thing, but measured in inches

Answer 3

measure the inside diameter of the rim in inches.