Size and Weight:
Discovered in inner Mongolia by the famous paleontologist Paul Sereno, Raptorex lived about 60 million years before its more famous descendant Tyrannosaurus Rex--but this dinosaur already had the basic tyrannosaur body plan (big head, powerful legs, stunted arms), albeit in a diminutive package of only 150 pounds or so. (Based on an analysis of its bones, the sole specimen of Raptorex appears to have been a full-grown adult six years of age). Analogizing from other early tyrannosaurs--like the Asian Dilong--Raptorex may have been covered with feathers, though as yet there's no definitive proof for this.
A recent study of Raptorex's "type fossil" has cast some doubt on the conclusions reached by Sereno. Another team of paleontologists claims that the sediments Raptorex was found in have been dated incorrectly, and that this dinosaur was actually a juvenile of the late Cretaceous tyrannosaur Tarbosaurus!