When mariners tell tails of giant squid so huge they can devour ships, they are rightly dismissed as tall tales. When they tell a story about a squid that was "about as big as a person but with really long tentacles", I don't think that ever got nearly as much ridicule.
They are much larger than a man, both in size and weight. And that's using those who have been caught. They are large enough to attack whales and small ships. I think there's more mythical creatures to be found in the ocean, specifically giant sharks.