Interesting question, and depends on the type of carb to some extent - eg whether slide type or CV. I'm much more familiar with the former. Essentially the smaller the bore the faster the air / fuel mixture passes through it. So a small bore will give easier starting and probably better running at small to medium throttle openings. Larger bore will be harder to start, be less flexible but give more top end power. And of course we need to consider the rest of the inlet (end indeed exhaust) tract as there's no point in putting a humonguous carb where the inlet tract is narrow and tortuous.
Phil Irving suggested that for racing purposes 1.1/8" - 1.5/16" would be about right for a 500cc single eg Manx Norton. Although this was in the 1950s, those sizes may be considered small now as maximum engine speeds have increased.
I have fitted a 22mm Chinese Mikuni copy to my Grumph (350cc Triumph twin engine) but as yet have not started it. Standard carb is a 20mm Amal monobloc which is pretty tiny.....
