Coolant overflow capacity?

A vented overflow tank can be as low as you want but the end of the hose from the radiator cap (or if the tank has a hole in the bottom) should be covered with coolant so it will siphon back into the radiator after it cools down (must also have the rubber seal in the radiator cap for that to work well). Completely filling the radiator and using an overflow or expansion tank also adds coolant to the total system volume.