Commonly 7" to 10" but it's not unusual for them to reach larger. Frequently up to 16". The IGFA record is almost 20" and 3 lbs. So the question becomes do you assume you'll have a small fish that may actually be stunted because you kept it in a too small tank or do you try to give the fish it's best chance to grow as much as possible by providing larger quarters. I vote for the latter and advise against keeping this species in a 55 gallon tank.one site said 7-10". i guess that was wrong.