IMO, a 20 long is a good starter size. Even fish like tangs can survive the short confines of QT in a tank of this size. Try to factor in the type of fish you want to keep, as well as their adult size. If you plan to keep larger angels, tangs, tirggers, etc, then go larger. I use a 40 breeder, which works well even for my more sizeable fish. If you're looking to keep small clowns, gobies, etc, then a tank your size is fine.