15-20 gallons is a decent size. It really depends on how many fish you decide to purchase and once and how large they are. Many people will just use a simple 10g aquarium or rubbermaid container and just purchase one fish at a time. With aquariums this small you just must make sure to keep an eye on water paramenters during the Qt
process. A fair amount of people will take down the Q tank in between fish as well so you don't have this empty tank sitting around collecting dust, and just refill it with water from the display when a new fish is purchased.