Stegosaurus was up to 26-30 feet long (8-9 m), about 9 feet tall (2.75 m), and weighed about 6,800 pounds (3100 kg). Its small brain was only the size of a walnut (weighing roughly 2.5 - 2.9 ounces (70 - 80 grams). Its skull was long. pointed, and narrow; it had a toothless beak and small cheek teeth. Its head was carried close to the ground, probably no more than 3 feet (1 m) high.
Stegosaurus had 17 bony plates that were embedded in its back. The arrangement of these plates was unknown until a 1992 Stegosaurus find in Colorado (Carpenter, et al.) - the plates ran along the Stegosaurus' back and tail in two rows, and the plates alternated in alignment.
The function of these plates is uncertain. The plates were made of bone which was not solid, but was filled with tube-like tunnels. The largest of these triangular plates was about 2.5 ft (76 cm) tall and just as long. The plates were probably well-nourished by blood vessels, indicating that the plates may have been used to regulate the dinosaur's temperature. They may have also been used for protection or mating display purposes.
Stegosaurus also had spikes at the end of its flexible tail (these are called thagomizers; they were named for a Gary Larson "Far Side" cartoon in which a caveman is explaining that the end of a Stegosaurus' tail is called a thagomizer, named for the late Thag Simmons). These spikes were up to four feet long and were used for protection from predators; they pointed to the sides of the tail. Different species of Stegosaurus had different numbers of tail spikes; Stegosaurus ungulatis had 8 spikes and Stegosaurus stenops had 4 spikes. Some early Stegosaurus had shoulder spines.
As even more protection, Stegosaurus had armor-like scutes on the skin of the neck, the pelvic area (the hips) and perhaps on the sides of some species. These bony scutes were
discovered by Carpenter, et. al. in 1992.
Stegosaurus' rear legs were longer and straighter than its front legs, which sprawled out to the sides. The forefeet (the feet of the front legs) had five short, wide toes with short, hoof-like tips. The rear feet had three short, wide toes with hooves.