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 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 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 ornithopod), and the enormous, sickle-shaped claws on its hind feet that it used to rip apart larger dinosaurs. We can thank the famous paleontologist John H. Ostrom, who discovered the first 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 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 (as it was portrayed in Jurassic Park).