I want to add a ball valve to my fuel line. I was thinking about adding it between the hose barb and the cool fuel III fitting. Does anyone know what size ball valve I would need?

the answer is 3/8.

mine was 1/2

what ever size you use,3/8 or 1/2 make sure it is a full-flow type.this is easy to ID.look through the valve and make sure the diamenter stays the same through the valve.