For those asking about shorter belts.
Both v-belts and micro-v (serpentine) belts are made to a set of standard cross-sections in Vs they are like A, B, C, D or measured in inches across the outer face of the belt.
V belts
Notes on Pulleys and Belts
micro
J Section Micro Rib Belts
I am not endorsing this second one they just have a nice diagram.
I used to work for a big name belt maker and we made every profile in lengths from around 8" circumference up to 144" in half inch increments.
The auto parts store should have a set of notched templates that fit into the pulleys and another for measuring the belts.
I have one around here somewhere and will upload a pic if I find it.