Stegosaurus was the largest stegosaur , reaching up to 12 m ( 39.4 ft ) in length and weighing up to 5,000 kg ( 5.5 short tons ) . Stegosaurus was the largest stegosaur , reaching up to 12 meters ( 39 ft ) in length and possibly weighing up to 5,000 kilograms ( 5.5 short tons ) .