I would take it wherever you have them made up and tell them to match it. That way there will be no confusion.
For the banjo bolt, I think it works the same as a regular bolt, ie if the hole is 7/16 the banjo bolt is 7/16. I just took my calipers for my 44 in the hardware store and started trying bolts until one threaded in, and then I bought that size banjo bolt.