Take the belt off, and loosen the motor bracket so that the bolt that holds it to the machine is in the center of the long slot of the bracket, then tighten the bolt.
Then, grab a piece of string, and wrap it around the motor pulley and the hand wheel pulley, and mark where the end of the string meets the rest of the string. Measure that distance, and it will give you a good approximation of how long the belt needs to be.
Your belt tension should be the same whether you are sewing thin or thick material.
When I put on a new belt, I loosen the motor bracket and put the new belt on, and just let the weight of the motor pull the belt taut, and then I tighten the motor bracket bolt, which often tightens the belt a little. Then I run the machine and do some test sewing, and if the machine slips, I tighten the belt a little.