Size and Weight:
Probably because it was so small, Microraptor fossilized unusually well--paleontologists in China have unearthed about two dozen more-or-less complete specimens of this tiny, feathered raptor of the early Cretaceous period, complete with traces of internal organs and primitive feathers.
The most spectacular fact about Microraptor is that it had not one, but two, sets of primitive wings--one on its forearms (similar to other birdlike dinosaurs, like Archaeopteryx), and one on its hind legs. This formidable feathered arsenal notwithstanding, scientists believe Microraptor was, at best, an occasional glider, much like a flying squirrel--and probably spent most of its life high up in the branches of trees. (A recent study has shown that the feathers of Microraptor were black and glossy, and likely evolved more as a way to attract mates than to fly the short distances between trees; an even more recent study concludes that Microraptor, far from being a clumsy flyer, was capable of swooping down from trees and plucking fish out of the water.)
This leads to the important question: was Microraptor a crucial "missing link" in the gradual evolution of dinosaurs into birds, or did it represent a four-winged experiment that (literally) never quite got off the ground? The answer may await future fossil discoveries, but the lack of any four-winged birds living today should give you an important clue.