Largest sea monster in a videogame

Described as a carnivorous, deep-sea fish, possessing of a cavernous mouth full of barbed fangs, the terrifying Black Leviathan from Man O' War: Corsair is the stuff of virtual nightmares for any sea-faring gamer. Based on the game's own realistic in-game scale, the creature would measure approximately 344 metres in length and 173 metres high should it exist in the real world, with a further length of 234 metres running from one hulking fin to the other. Man O' War: Corsair is a naval combat adventure, inspired by the cult Games Workshop table-top game. It is developed by Evil Twin Artworks and was released as Early Access in April 2016.

Hydra from God of War (2005) would run the Black Leviathan very close for size, but given that we see very little of the monster's body during the intense boss-fights with Kratos (besides the moment when Kratos runs deep inside its corpse to retrieve a key), it's very difficult to estimate its size with any degree of accuracy.