How big is your tank to Quarantine new or sick fish.

I'm looking for a tank to Quarantine new or sick fish. What is a good size tank to use. The other question I have is if you fine a sick fish in your tank will it do any good to Quarantine it. Ihope this issen't to dumb of a question

Thanks Rick

this probably depends on fish size. My side tank is a 20g. But my largest fish is around 5".

Same here, a 20 gallon.

i had a ten gallon. but im into the smaller cichlids. recently gave up on it and made a shrimp setup lol.