A couple things to keep in mind -
1. Pedersoli has been making these things for awhile. There could be some variation over time of the bore diameter.
2. Powder fouling will build up and make it hard to load if you use too large of a ball. All things equal, I'd go for too small of a ball before I'd go too large. If it ends up too sloppy, use two patches and shoot them up, then try a size bigger next time.
When I owned mine, I used .715 balls that I got from Track of the Wolf. I've still got a bag of them waiting for me to get another musket some day. They worked fine whether I was using patches or the college newspaper as musket wadding. I had no trouble loading, but I only shot 10 or so shots per range session. Between the flash in the pan and rudimentary sights, accuracy in mine wasn't great... but if I'd practiced the potential was there to maybe use it out to forty or fifty yards for hunting.