The general rule of thumb for O rings is to use an O ring 1/16" in diameter less than your tennon.
The O rings should be 1/16" thick as well.
So, for a 5/8" tennon, you would use an O ring that is 9/16" in diameter and 1/16 thick.
O rings are normally sized by SAE number, so for a 5/8" tennon you would want an SAE 013 O ring.
The o rings you buy at the big box store are Bunna O rings. They are horribly over priced and do not hold up well. For calls, you really want to use a Neoprene O ring. They last longer, and have greater resistance to chemicals and heat and cold.
We stock them in the forum store. Just click the banner below.
Al @ THO Game Calls