If you are thinking round ball, 0.678" is good for most standard shotcups and should accommodate a choke if it meets one. 0.662" works well too but is a bit small for standard shotcups ~ I "patch" mine to fit and it is quite accurate from smoothbore. Not sure how those would work in a rifled gun.
I have not found a ball/shotcup combination that worked for me using 0.690" balls. I find them too big for most shotcups. Some people have had good results with 0.690" RB in shotcups in rifled guns though. Do a search and you should find some info.
0.735" moulds are available from Lyman and I have had good results using ACWW balls over a hard card wad column in both smoothbore and rifled gun. I tried cushion legs, fiber wads and other methods but with poor results. A plastic gas seal, hard card wad column then the ball works well for me. By the way, that 0.735" RB weighs 580 grs. so not a lightweight. I like round balls!
If you are using a rifled gun there are a number of solid slug designs around as well and Accurate Molds has a good selection from 570 to 900 grs.
Greg: Have you shot a critter with that hollow copper slug? I read about a hollow copper bullet of similar design many years ago. The idea was hyper velocity defensive loads for .357 mag. (IIRC) and cutting a "core" all the way through a "bad guy". Makes me shudder to think of the wound channel!
Longbow