Framerguy has it right. I would use either (1) 1 1/2", (1) 1 1/4", or (2) 1", All three options would do the trick, unless you have zero head loss, then you will have to use the 1 1/2 or 1 1/4(might be hard to find a bulkhead that size, IDK, I've only ever used 1", 1.5", and 2"). What size...