I hope this pipe dimensions
chart helps with what you are looking for. If you don't already have a pair of calipers, buy them.
I used to have a few glowsticks, 14" long, and 0.585 inches thick. I considered a 1/2 inch (0.622) pipe good enough to shoot them from.
What I consider a perfect fit, is something that you need to push a little for them to fit in the barrel.
I once bought round .50 caliber lead air rifle pellets that were 0.495-0.496 inches. What I used was 3/8 inch pipe, which was 0.493 inches in inside diameter. If you have calipers, take them out, and try to measure out 0.005 inches; that's how much larger the lead balls were than the barrel. This
is a perfect fit.
I also had shaped lead pellets, 275 grain, and 0.4995 OD. They made a much tighter fit in the barrel. I preferred these for their heavier weight and tighter fit. Those things weighed 18 grams, but damn did they fly fast!
Link to the 275 grain pellets
They also sell the 0.0045 inch smaller balls if you're interested.