You measure the diameter of the drum where the line wraps, not the flange at the top. I got a cheap plastic caliber in a hardware store that is good for that job. I set the caliper to the right size on the winch and then measure it with a tape measure. I dont trust the calibration on the caliper itself.
When you start to put your winchers on you will think you got the wrong size. They fit very tight are can be a real challenge to force on the winches. They do help.