I keep a couple of old belts cut in half. I wrap them around the pulleys with the generator adjusted to almost its lowest spot. Wrap the belt around there and measure the gap between the belt ends. Add that to the total length of the belt and go get one. if you're off a little bit you can always return it for one slightly longer or shorter belt. The length should be printed right on the belts paper part number. Buy the right "width" belt, I see people running narrow belts in wide grooves. Belt are designed to grip the sides of the pulleys not the bottom of them. We must have had 100 belts at work hanging on the wall for the fleet. Besides being a mechanic, I was in charge of the parts department for the phone company for 30-years. We worked nights so there was nowhere to get parts, you'd better have them in stock, or the vehicle could be out of service.
If you don't have a cut belt you can try a piece of masking tape or something like it to wrap around the pullies to get a measurement. It's starting point. The belts are listed online in the Gates or Dayco catalogs. There organized by width and length so it's easy to find a part number. We changed all the catalogs out every year, so I took the old ones, I have a collection out in the garage.
