Great Whites... and REALLY Great Whites

Everyone's fascinated by the great white shark, a beast that can be 7m long and can (and has) put humans on its menu on occasion. It is the largest and, in some ways, most evolved of a line that goes back 400 million years. Sharks outlasted the mighty Dunkleosteus, the great reptiles like Liopleurodon, and the most fearsome whale of all time, Livyatan melvillei. Today there are nearly 400 known species of shark, and the great white is their metaphorical king (or queen, since the biggest ones are always female).
One thing people like to speculate on is how big great whites get. The maximum length of the species has been subject to countless tall tales, overestimates, and mistakes (for a long time the record was 36.5 feet / 11.1m, but this was a misidentified basking shark.) Looking at claimants from the Azores, Cuba, and Australia, the answer seems to be that the provable maximum length is under 23 feet / 7m. (I've named 7m the "Ellis/McCosker limit," since Richard Ellis and John McCosker have done the most research on this and produced the best single book on the species so far.). There is a case where an expert theorized bites on a whale carcass floating off Australia might belong to a monster in the 25-foot neighborhood (around 7.5m) which would be mean real great whites get as big as Bruce in Jaws.  The question of exact lengths can devolve into a pointless debate over inches/centimeters, so I'm going to say the biggest great whites approach 7m, with exceptional sharks possibly larger, and call it a day.

Katherine, a great white tagged off the Florida coast. (Wikimedia Commons)

For centuries, human being killed great whites when they had the chance and otherwise avoided them at all costs. Silly stories about a shark swallowing a whole man in armor did nothing for their reputation. Herman Melville called the great white "the dotard lethargic and dull, pale ravener of horrible meat."
Today, scientists take significant risks in order to learn more about great whites - not to kill them, but to conserve them. This article from Nat Geo tells of a dramatic tagging venture, involving the largest great white ever pulled out alive, tagged, and returned to the ocean. "Apache" is 5.5m (18 feet) long, enormous for a male. the shark weighs about two tons and did not come aboard peacefully, but the conservation team lead by Michael Domeier looks for a selection of sharks, including the biggest (presumably oldest) as well as the smaller ones that are relatively easy to handle. (Relatively.)
There's a lot we don't know about this species, including its mating and migration habits. Without knowing more, we can't tell whether the population is being affected by factors like climate change and overfishing of its prey. Great whites are taken in the pointless and destructive shark-finning trade, but, though classed by the IUCN as Vulnerable, they are not endangered - at least not yet. This is important because healthy top predators are vital to an ecosystem. This article documents how at least one population seems to actually be on the rise.
Those of us who dabble in either sharks or cryptozoology are always asked about Megalodon: Carcharocles megalodon, a distant cousin of the great white, which became extinct over 2m years ago.   While Meg gets 80 - 200 feet long in fiction, we know for sure that they reached at least 15m, maybe 18m, and may have been the largest fish ever to have lived (there's quite a lot of dispute about a couple of older prehistoric fish, but there is no doubt Meg was  at least the largest shark ever.)  

Megalodon naturally attracts novelists like, well, great whites are attracted to chum.  A lot of the literature is fun, if little of it could be called scientifically precise. Steve Alten has made a good living off his fictional Megs. Briar Lee Mitchell write a good novel with the all-time-best title of Big Ass Shark.  There are lesser-known novels, incredibly bad cheap-crap movies, and even worse fake "documentaries," along with other ways to get a Meg fix, but the fish is extinct, period, done, over with, gone, dead. the only really interesting possible Meg sighting, which novelists have played off a good deal, was the New Zealand shark of 1918 claimed by lobstermen to be ghostly white and at least 100 feet long. This case was accepted by an expert who interviewed the men, and it's frankly still a mystery: if I had to make a guess, I would suggest an exceptionally huge and unusually light-colored great white, plus human exaggeration factor, was involved.  (Ellis once observed that the name "great white" only makes sense if you're looking at the shark upside-down.) 

So no Meg, but we'll settle for the great white - which is more than enough. It's pretty damn awesome, and hopefully we can keep it on Earth for millions of years to come.  

References: 

Bright, Michael.  1989.  There are Giants in the Sea. Robson Books.

Civard-Racinais, Alexandrine, and Maud Fontenoy. 2012. Great White Shark: Myth and Reality. Firefly.  

Compageno, Leonard, with Marc Dando and Sarah Fowler. 2005. Sharks of the World. Princeton.

Ellis, Richard, and John E. McCosker.  1991. Great White Shark. HarperCollins.

Ellis, Richard.  1983.  The Book of Sharks.  Alfred A. Knopf.

Ellis, Richard. 2012. Shark: A Visual History. Lyons Press.

Klimley, A. Peter. 2003. The Secret Life of Sharks.Simon & Schuster.

Lineaweaver, Thomas H., and Richard H. Backus.  1970. The Natural History of Sharks. J. B. Lippincott.

McCormick, Harold W., et. al.  1978. Shadows in the Sea: the Sharks, Skates and Rays. Stein and Day.

Ricciuti, Edward.  1973.  Killers of the Seas.  Collier Books. 

Steel, Rodney. 1985. Sharks of the World. Facts on File.

Wood, Gerald L. 1977. Animal Facts and Feats. Sterling Publishing Co.

"Great White Sharks," http://marinebio.org/species.asp?id=38

"Size of the Great White Shark," Science, 13 July 1973.