a) Rubber/rubber like belts shrink in diameter, but increase in circumference/thickness. So belt tension tends to increase as this is a double-whammy in practice. But, a brand new belt will have some initial give. The rule-of-thumb is deduct no less than 5%, no more than 10% from the diameter of the old belt. The longer the belt, the higher the deduct.

b) Belts are cheap, mostly less than 1 US$ each. Bracket the size if in doubt.

Peter Wieck
Melrose Park, PA