Yup, definitely use the link above for Bob's Black Powder Notebook, lot's of good info there, and Bob is a heck of a great guy, very helpful with a great source/system of documentation.
I have two smoothies, both 20 gauge. My Tule Fusil de Chase has a bore measuring .618 and it shoots .600 ball patched with .010 patches very well, it was the best combo I tested, and is fairly easy to load. My Early Virginia Smooth Rifle has a bore that measures .615 and the afore mentioned patch and ball combo is very hard to load and doesn't group too well, with any powder charge tried. A .600 ball sitting on a lubed 1/4" felt wad groups better, and a .615 ball sitting on the same wad over two thin cards separating the wad from 85 grains of 3f shoots best.
Point being, read what Bob and others suggest, but each gun has it's own appetite and it will take some experimenting on your part to find its optimum diet.