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. read more
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. read more