Size and Weight:
If you saw a picture of it without any sense of scale, you might find Compsognathus every bit as scary as its fellow theropod Tyrannosaurus Rex, with its sharp teeth, big eyes, and predatory stance--so you might be relieved to know that this late Jurassic dinosaur was no bigger than your average chicken, only about two feet long and five pounds soaking wet (See also 10 Facts About Compsognathus.)
Once thought to be the smallest dinosaur that ever lived, Compsognathus has since been bested by the even tinier Microraptor. But this "pretty jaw" was not to be taken lightly: paleontologists believe Compsognathus was blazingly fast, with good stereoscopic vision, and occasionally capable (perhaps) of taking down larger prey. As is the case with many small theropods, there's a strong possibility that Compsognathus hunted by night, in sizable packs, so as to stay out of the way of the larger theropods of its ecosystem (notably the three-ton Allosaurus).
One controversy about Compsognathus that hasn't been settled yet is whether or not this early theropod sported primitive feathers, like so many others of its breed. No trace of feathers has been found in existing Compsognathus fossils, but then again, a soft, downy coat would have left little or no imprint in the fossil record--and a feathered Compsognathus would fit in better with what we know about the small "dino-birds" that lived tens of millions of years later, during the Cretaceous period.