Megalodon is the largest carnivorous fish known to have existed, and quite possibly the largest shark to have ever lived. It is believed that the average size of the megalodon was between 12 metres (39 ft) and 18 metres (59 ft). But the size of a modern female blue whale is over 30 mtrs (115 feet) which makes so far as we know the largest animal that has ever existed. ( Until we find fossils of something bigger). So the Megalodon was probably bigger than some smaller whales but not the Blue whale. Also since sharks don't have a bony skeleton this size is an estimation based on the assumption that the size to tooth ratio is the same of present large sharks but this could be in error. Also Despite popular beliefs and myths, the Megaladon might have never existed. I think it did exist though, but either way.. The blue whale shark is the largest animal to have ever existed (as of right now of course :p) and is still extant. The Megaladon is thought to be the largest of most sharks, bigger than the Great White shark. Unfortunately, it's extinct and we still don't have much knowledge on its certain qualities, features, etc, etc