Most paint these days is in the .677 to .685 range, so you should get one in that range. Check the paint that you use most often with barrels or inserts of different sizes. The ball should not roll out easily, but come out of the barrel if you blow through it gently. It is probably better go go slightly too big than too small.
"For doubt and secrecy are the lure of lures, and no new horror can be more terrible than the daily torture of the commonplace."- H.P.Lovecraft "Ex Oblivione"