Autozone sells some really cheap belts, like $4 or so, go buy one thats way to long and cut it to use as a measuring tool. I have a 1/2" belt I use when I need belts. Just wrap it around the pulleys and use a piece of tape to mark where they over lap. It will get you really close to the size you need and is reusable.
Just a word about belt measurements, Gates
https://www.gates.com/us/en/power-transmission/v-belts/automotive-v-belts.html and others use the middle of the belt for the size. The
effective length is not the same as outer circumference and its not the same as the inner circumference, its halfway between. Parts stores may use either. My cut belt measurement will get you the outer circumference and with some math, you can work out the effective length.
Use the Gates link above to narrow down your search.
