There are a few measurements that you need to be aware of regarding any trailer. To get the measurement, it's easiest if you remove your tires and wheels, or measure center of tire to center of tire. It can also be helpful to have a friend hold one side of the tape measure, but it’s not absolutely necessary.
Hub Face Measurement:
To obtain the hub face measurement, first hook your tape measure to the outside of the hub, (the part of the hub facing the wheel). Measure across the axle to the exact same spot on the opposite side. Be careful not to measure to the inside of the hub, only the outside. This is the primary measurement that is needed. The rest of the measurements below are alternative measurements that can be used to arrive at this measurement.
Spring Center Measurement:
This measurement is critical. If the springs are misplaced due to bad measurement, you can end up bending the axles. This one is difficult to do alone. Have a friend hold the end of the tape measure right in the middle of one of the springs, while you extend to the center of the opposite spring. If taking the measurements on your own, then you can cheat by first hooking the tape measure on the outside of one spring and then extending it to the inside of the opposite spring.