That measuremnt sounds right for nema 23 motors. You can do a google search and find the exact measuremnt of each size motor. The mounting flange is what they go by for frame size ( Nema23 - Nema 17 and so on are different frame size, mounting flages). The lenth will effect the power. All though all motors are not equal. You can have a shorter motor same frame size that has more power than a longer motor. But normally if they are same series motors the longer, the more power. If they run the machine it really doesnt matter. And sometimes smaller motors do just work better depending on the setup of the machine and motor design.