Do you currently ride a bike? Usually, you base your next bike size on what you are currently riding, +- any differences you want to make.
If you currently dont ride, then it is a lot more difficult. A ballpark figure is to leave x" of clearance between your 'nads and the top tube, where x varies with the use (road, MTB), and style (horizontal top tube, sloping/compact top tube).
A simple measurement of frame "size" is from the bottom bracket to some point on the top tube. The BB itself can vary in hight off the ground. The same "size" frame may vary by up to 2" in its standover clearance.
Given adaquate standover clearance, then the really important dimension is the length of the frame, which affects your reach. This depends on your body proportions and on your riding style, ie how low and aerodynamic or upright and comfortable YOU like to ride.