Last week, a paper was published in PLOS Biology by a team of Oxford scientists who, based on skeletal measurements, calculated the masses of 426 species of dinosaur. The largest, Argentinosaurus, was a massive sauropod weighing in at roughly 90 tons, while the smallest they measured was Qiliania, an Enantiornithine bird that weighed 15 grams, making the weight of the largest dinosaur 6 million times the weight of the smallest non-extant known species (the smallest-ever dinosaur known is the Bee Hummingbird of Cuba which weighs 2 grams). It's suspected that size played a large role (no pun intended) in the survival of the maniraptorans past the mass extinction 65 million years ago, as the smallest dinosaurs may have been able to avoid the worst effects of the asteroid impact by being more generalist, requiring fewer resources, and/or being able to exploit niches not available to larger species.
Update: today (5/16/14) a team of paleontologists in Argentina announced they've found a new sauropod species that may be even bigger than Argentinosaurus, and estimate its weight at 100 tons. Should be interesting to see what comes of this new discovery.