Size: Estimates range from 50 feet, all the way up to 90 feet in length. It would also weigh somewhere between 60 to 100 metric tons. (One metric ton is 2000 lbs.)
Appearance: Megalodon is a shark, similar to a Great White, but many times bigger.
Threat: High. Megalodon is a predator, and while it would undoubtedly prefer whales or other large animals, anything that fits in its mouth is fair game. This is also an animal that would have no problem bringing down a large boat, so it is easily conceivable that it's existence is being kept a secret due to the fact that there are no survivors from sightings of it.
Sharks have been feared ever since humanity has set out in boats and see their large shapes circling them out of curiosity. Just the idea of a twenty foot shark is enough to send most people scrambling for the beach. Imagine if people knew there was a sixty foot beast swimming around.
Animal Planet's recent docu-drama on Megalodon, like their pieces on mermaids, has stirred additional controversy, with many people completely unaware they were watching a dramatization and not really seeing proof that the giant shark is still swimming around, eating whales.
While it would really be something to have Megalodon swimming around these days, there's one big reason it's not. Lack of food. Sure, for one or two Megalodon, there might be enough whales around, but you need more than two for a viable breeding population. That might not be up in the hundreds, but even to have just 50 Megalodon's swimming around, you would need much more full oceans than we currently have. As it is, whales are well known, and some species are rebounding from the way they were over-fished thirty or forty years ago, but they are still not common. In addition, ocean populations around the world are steadily declining from over-fishing and pollution. So there really is no way a breeding population of Megalodon's would have enough food to survive today.
Of course, if you're interested in seeing giant sharks, there's no shortage of movies. JAWS is still the best example, even though it's not a Megalodon. In addition to that are whole series, like the Megashark Vs. series, and the Shark Attack films. For one-off's, you can watch Jurassic Shark, Megalodon, and Dinoshark. How much I would recommend most of them outside of JAWS though is very debatable.
If you're looking for books. Of course you can pick up the original story of JAWS by Peter Benchley. Then there is also the Meg series by Steve Alten. So there is lots of entertainment to pick from.