Stegosaurus is a very famous dinosaur. It's also very large. They were up to 23-25 feet long and 3-4 tons in weight. These dinosaurs had large tail spikes that were up to 3 feet long. With a brain the size of a golf ball, Stegosaurus had one of the smallest brain to body ratios of any animal. Stegosaurus lived during the late Jurassic 150 million years ago in North America and possibly Europe.