Denso belt suggestion

Automotive V belts are measured buy the circumference around the outside of the belt.

(As opposed to industrial A or B belts, which are measured by the "core" for lack of better term)

So for a starter just run a flexible tape around the outside rim of the pulleys. It helps if you have a friendly parts store than can temporarily hold an invoice for say, 3 belts of various lengths

Most automotive belts part numbers now have the belt length encoded in the part number. In 74, when I first "got into" auto parts, Gates belts were not that way.