If you have a tape measure like seamstresses use, you can position all your pulleys where they will run and wrap the tape around them on the outer edge of the sheaves so you know what size belt to search for.
V belts are generally sized by the inner circumference but if you take your tape with you, you can measure the outside circumference at the store.
Or you can do math involving pi to calculate the inner circumference based on the thickness of the belt.
