There's little doubt that the largest animals ever to live on land were sauropods, which includes Apatosaurus (formerly "Brontosaurus"), Diplodocus, and Brachiosaurus. But deciding which was the largest depends on what one means—longest or heaviest? Based on fairly complete skeletons, the sauropods Argentinosaurus and Seismosaurus, which were up to 50 meters long, are the longest dinosaurs yet discovered.
Among dinosaurs that aren't birds, or nonavian dinosaurs, the Compsognathus longus, an Early Jurassic, chicken-sized meat-eater from Western Europe is the smallest yet discovered. It measures only 70 centimeters long, including the tail, and probably weighed about three kilograms.