It really depends on your application. If you are in doubt, tie some strings so the length is equal to the starter belts, & try them out to see which one you prefer.
Try 15" for .21's, 17" for .45-.60's and the 19" over .60 sized engines.
Some things to look at are if the engine has a side or rear exhaust and if the hull is a low profile or deeper like an offshore boat. Too long is better than too short.
If you are talking about riggers, I use a 13" belt from Aeromarine ( just like prathers and the smallest one I can find) on my .12 and a 17" for my .67.
We were at the pond one time and a frined had an A-mono with one of those monster size belts that hung out of the boat. He snagged part of the booey with it and it sling shot the boat right around it flinging it in the air and turned it upside down. Funniest thing I ever saw