48 Frame Motor = 3 - 5/8"
56 Frame Motor = 4 - 1/8"
The easiest way to determine if your hot tub pump / motor is a 48 frame or a 56 frame, is to measure the distance between the through bolts. Measure across or down, but not diagonal.
The through bolts run from the back of the motor, go through the full length of the motor, and connect to the plastic pump wet end. If your wet end is installed on the motor, you can also measure the distance between the bolt heads at the rear if the motor. If you can't see the bolt heads at the rear of the motor, you may have to remove the plastic rear cover that covers the electrical. To prevent getting electrocuted, make sure you turn off all power before removing the electrical cover!
If you measure 3-5/8" between 2 of the bolts, you have a 48 frame pump and motor. If you measure 4-1/8" between 2 of the bolts, then you have a 56 frame pump and motor.
Measuring is the best way to determine if you have a 48 or 56 frame pump. You can also look at the Name Plate sticker on the pump, however, the sticker can often be misleading.