You probably could get away with 1/2" full flow ball valves (1/2" i.d.) but I wouldn't go any smaller. There aren't any 5/8" Ball valves, so you'd have to bump it up to 3/4". I believe a 5/8 hole is roughly 60% larger than a 1/2" hole (area wise). While a 3/4" ball valve is overkill, I don't know what the I.D. of the fittings are that you need to get back to 5/8" I.D. air line.
