One of the guys in the archery department of Cabelas gave me this advice for getting the correct consistent sizing for string loops:

Take a new string, cut one end, fray it and melt the end and make a "ball" out of it. Then lay the string flat, measure 4 3/4 inches (I have it written down at home, but I think this is the correct size) cut that end. Once again, fray it, melt it and make a ball. You now have the correct size and it will always be consistent if you change later on.

Takes the guess work out of it. At least for me it did and the size seems right.