Did you copy and past that proto?
Anyway , the first question. There is no real pro/con to the different bore size. Some balls are smaller than others, and some are bigger. So the bore that fits the ball better is the best choice.
What you need to do, is fine a bore that will fit the ball almost perfectly. If you pick a bore to small though, the ball will shoot inaccurately. If you pick a bore to big, the ball will shoot not accurate as possible.
I personally put the ball in the bore, and blow into the barrel. If it shoots out without problems, that is what bore I use.