Size and Weight:
Of all the dino-bird fossils discovered in the Liaoning Quarry in China, Sinornithosaurus may be the most famous, because it's the most complete: its perfectly preserved skeleton shows evidence not only of feathers, but of different kinds of feathers on different parts of its body. The feathers on this small theropod's head were short and hairlike, but the ones on its arms and tail were long and distinctively birdlike, with feathers of intermediate length along its back.
Technically, Sinornithosaurus is classified as a dromaeosaur (raptor to you and me), based on the oversized, sickle-shaped single claws on each of its hind feet. However, it bears a greater resemblance to other dino-birds (like Archaeopteryx and Incisivosaurus) than to famous raptors like Deinonychus or Velociraptor.
The latest news about Sinornithosaurus (as of December 2009) is that it may well have been a venomous dinosaur, as evidenced by fossilized pouches found connected by ducts to its snake-like fangs. Judging by the anatomy of modern animals, it would be surprising if these sacs weren't exactly what they appear to be--repositories of venom that this feathered dinosaur used to immobilize (or kill) its prey. (Hold the presses: a more recent study concludes that the "pouches" of Sinornithosaurus were created when this individual's incisors loosened from their sockets, and aren't evidence of a venomous lifestyle after all.)