The "official" recommendation I've seen is 21/64". But rather than track one down I used a 5/16" and very judiciously sanded for the difference. Actually worked better for than one a friend made using a 21/64". I like mine fairly snug, and for my tastes his are just a little too loose. Might be just fine with heavier patching, but my rifle prefers .015 rather than .018 or thicker. Lots of wiggle room in the dimension I think, based on how tight you want the block and the thickness of your patches.
