I was thinking of starting with a .675 and .680. If I need anything smaller i can get that in the future. I had some paint that was sub .670 this summer but it was crap JT I bought at Wally World when I was in a pinch for testing a marker.
While I suppose you could use an insert bigger than the bore of the barrel, I guess it wouldn't matter as long as the paint would get through the .680 barrel. To keep things simple though, a .680 insert should be the biggest I need.