You'll want to mic your bore when you get it, before ordering any balls or a mould. Your barrel will be off, give or take a few thou", from whatever a 16 gauge is supposed to be. My .62 calibre Colrain barrel is actually a .612".
My smoothie likes a tight ball/patch combo for accuracy, but "tight" in a smoothie is a little loosey compared to a rifle, as when the barrel fouls it gets a little harder to load than a rifle barrel. I load my first load with a .018" patch, and proceding rounds with a .010".
For a comparison my musket bore mics out at a true .750", best as I can tell, and a .735" ball gives me a real tight fit. With a Wonder wad, and Swiss powder I can get three shots for sure before wiping. That might give you a ball park figure, but again you won't know exactly what size your bore is until you get it.
Good luck and have fun, you will love that sweet sixteen.
Rat