Size and Weight:
If Deinonychus looks familiar, that's because it was popularized in the blockbuster movie Jurassic Park under the catchier name Velociraptor (real Velociraptors were actually much smaller than the fast, sleek, man-sized predators depicted in Steven Spielberg's movie). See 10 Facts About Deinonychus and a gallery of Deinonychus pictures
Although it was far from the biggest dinosaur of the late Cretaceous period, Deinonychus was especially fearsome because of its speed, its presumed ability to hunt in packs (tangled Deinonychus bones have been found in close proximity to the remains of Tenontosaurus, a tasty and slow-witted ornithopod), and the enormous, sickle-shaped claws on its hind feet that it used to rip apart larger prey. We can thank the famous paleontologist John H. Ostrom, who discovered the type specimen, for much of what we currently know about Deinonychus--as well as for the idea that raptors like Deinonychus eventually evolved into modern birds.
As is the case with other raptors, the actual appearance of Deinonychus is a matter of debate: today, it's often depicted as sporting primitive feathers, though its skin may well have been more reptilian in appearance, at least on portions of its body (as it was portrayed in Jurassic Park). As for the presumed intelligence of this dinosaur, that has been way overstated by Hollywood: there's no way Deinonychus could have turned a doorknob, as depicted in Jurassic Park, and in fact it could easily have been outwitted by a six-year-old child.