Getting a definitive answer for this question would be all but impossible. Several of the really huge species of fish keeps on growing all their lives, and some can get really old. Pretty much all of these huge species have been overfished to the brink of extinction, and so we probably won't see any properly grown specimens for years to come. And that is only so long as the relevant bans and laws are enforced. All we really have to go on is old tales and written accounts from centuries ago.