The Great White Shark has been known to science for centuries, and probably the most feared (though not the most dangerous) shark in the world. However, the maximum size of the Great White is still unknown.
Up until 1973, the largest great white was believed to have been 36.5 feet long (approaching the size of a Megaladon). Eventually, an examination of the jaws that had belonged to the shark revealed that the shark was actually 16.5 feet; the original measurement had been a typo.
A photograph in Richard Ellis's book, Great White Shark, shows a great white caught off Maldive Islands which was said to be 30 feet long, though there was no other proof with the photo. Jacques Coustea mentioned a 25 foot great white, and Daniel Webster referred to a 32 footer, neither of which had any confirmation. Then there is also the story of gigantic white shark seen by several fishermen (see Megaladon for more info) which was related to David Stead.
Currently, the record remains at 21 feet long. There was also a 23 foot great white reported to have been caught in the Mediternanean, and there may be bigger great whites out there.