Ideally, your water cooling system should only need enough water to fill the hoses and the radiator (and a small reservoir). Mine holds (I think) about 3/4 of a liter, but I've got a rather large radiator.
I've heard tell of people who have massive reservoirs that hold gallons of water.
IMHO, if you need a large reservoir to maintain reasonable temps, you haven't got a large enough radiator for the heat load you are pushing at full tilt.