They get around 4-6 inches depending on species - adults could probably munch on those tetras, although they should be fine together for a while. They're more comfortable in groups though, so 4-6 specimens would be ideal. 20 gallons is a bit on the small side, but again you could get away with it for a while. Plan on around the 36 gallon range for a few adults. They make great tankmates for medium sized gouramis, catfish, barbs, etc.