Size and Weight:
In many ways, Megalosaurus was your garden-variety theropod dinosaur, indistinguishable from a host of other big-headed, large-toothed carnivores of the late Jurassic period. What really sets this predator apart is the fact that it was the first dinosaur ever to be discovered and named: a fossilized femur of Megalosaurus was dug up in England in 1676, but it wasn't until 150 years later--after further discoveries--that it was given its name, Greek for "great lizard," by the early paleontologist William Buckland.
The early discovery of Megalosaurus did a lot for the burgeoning science of paleontology, but it had a detrimental effect as well--for decades afterward, every dinosaur fossil that even vaguely resembled Megalosaurus was assigned to its genus (this was before scientists had appreciated the vast diversity of dinosaurs during the Mesozoic Era). This confusion is still being sorted out, as various "species" of Megalosaurus are occasionally reexamined and assigned to their own genera.
By the way, Megalosaurus has the distinction of being the first dinosaur ever to appear in a work of fiction. Way before the Hollywood era, Charles Dickens name-dropped this dinosaur in his novel Bleak House: "It would not be wonderful to meet a Megalosaurus, forty feet long or so, waddling like an elephantine lizard up Holborn Hill."