Fuel capacity depends a lot on what you're doing. Big difference between aerobatic planes and warbirds. I have an extra with a 55cc engine, and 16 oz. is plenty. Mostly due to the light weight of the plane and the low throttle that is needed to fly it. It seldom needs over half throttle, unless you put it into a hover. Most warbirds are much heavier, and get ran at higher throttle settings. My 55cc warbirds all have 24oz. tanks. That gives about 8 min. with a couple min. of backup. If you go to big flying events you usually like a bit more backup fuel on board. It often happens that you have to extend your flight because of some incident on the runway or a backup of landings and takeoffs. For my Warbirds (fighters) I run 24 oz. for the 55cc, 32 oz. for 65-70cc. and 40 oz. for my over 100cc engines.
I think the stock tank is about 30 oz., but I'm always afraid to use stock ARF tanks because the seams often split.