Depends on the species, but I'd say a 30-40 gallon would be about fine. Go easy on the feedings, and it should stay relatively small (again, depending on species).
We have 2 anglers.. one is in a 48x24x12 (60 gallons) frag tank, and because the girls love to feed he's grown 2 inches in like 6 weeks. The other one is a baby scarlet frogfish that's currently in the 11 gallon counter tank. There are smaller species that I could see keeping in a 20-30 gallon tank, but if you feed the larger ones and they reach an 8" type size, then an appropriately larger tank is needed.
