How many gallons should a hospital tank be?

I have 4 - 5 inch goldfish (Fancy-type). They are in a 50 gallon tank. I don't have a hospital tank, in case any get sick. Up to now, I have had to treat the entire tank for illnesses. How many gallons should a hospital tank be for 4 - 5 inch Goldfish? How powerful of a filter should be on it? Once all signs of illness are gone, say Fungus for instance, do you put the goldfish back in main tank right away, or wait a few more days, or what? (NOTE: None are ill at this time.) Also, how do you get the hospital tank cycled when you need to use it? You don't always have it set up, do you?