At least a 10 gal should do unless you're planning on QT a lot of new fish at once or you have larger fish that wouldn't fit into a 10 gal if they got sick. I keep mine in the closet lol You could either run an extra filter on your main tank (even a sponge filter would do) that you could then pull out if needed for a QT or if you have a large filter (like a canister) you should be able to pull out some media for a QT tank without disrupting the main tank too much. New fish should be QT for about a month as some diseases can take that long to show up. I also have a spare heater and air pump handy for setting up QT tanks. Something in the tank to help the fish feel secure is a good idea (decorations, plants, etc), that's about all I put in mine.