.693 is the most common or 689 for shooting paint with a smaller bore such as proto but as summer is coming .693 is a good all rounder that allows for some swelling, its what most stock barrels are
Brilliant,light,shoot through,prutty....
Go large first as suggested 693 then build up a set to suit,they're pretty cheap just getting harder to find and you may have to go with whatever colours are available,I've got black,green and red.