Size and Weight:
All we know about Scipionyx is based on a single, spectacular fossil discovery: an almost perfectly preserved juvenile only a few inches long. Amazingly, paleontologists were able to "dissect" this fossil, revealing remnants of the unfortunate infant's windpipe, intestines, and (most importantly, from a metabolic perspective) its liver.
What do these internal organs tell us? Well, Scipionyx had shorter-than-expected intestines, leading biologists to conclude that it was unusually efficient at digesting its food (which probably consisted of insects and small animals). Also, the size, shape and color of this dinosaur's liver has provided an important clue to the organization of the average theropod's internal organs.
As to how this Scipionyx lived--or what it looked like fully grown--that's a matter of some speculation. Scipionyx was clearly a type of theropod--which puts it in the same carnivorous family as Tyrannosaurus Rex--and it may have sported primitive feathers; lately, paleontologists have classified it as a coelurosaur, and thus most closely related to the early dinosaur Coelurus.