[Christ]

Is it the cam belt too? If so, can you either make the tensioner more adjustable, or make a second one that will take up the slack?

If it's a plain belt, you can run a string around the drive pulley and the driven pulley, tie it off, take it back to the store, and they'll usually let you browse the full line of belts they carry, so you can find the right size. They may do the same for a timing belt, just ensure that the teeth are the correct distance between them.