This game uses I believe the C5161-1 batting unit with a metal bat that the linked charts say uses a 7/8" ball. The charts also indicate a 4693 pitching unit the same as the previous year game Four Bagger.

AlexF, has a great example of that game and is very helpful. He usually answers here. If you want PM him and he will tell you the number of balls he has in his game and confirm it is a 7/8" ball.

https://pinside.com/pinball/forum/topic/williams-baseball-game-motor-coils-and-bat-assembly-information#post-4333051

Did you ever find out the answer to this? I figure it is 7/8". I don't know how many, though. Thanks.

Mine was actually a 4 Bagger. The model that came out a year before. I since sold it but think 7/8 is correct. As far as number of balls goes. You could probably get by with as little as 3 but might experience some lag in gameplay. 5-7 is probably better and you could even have more if you wanted the way they just filter down to the pitching mech.