Dicraeosaurus was a medium-sized Sauropod. Calling an animal this large a medium-sized anything says something about just how large the Sauropods were.
Fossils indicate that Dicraeosaurus may have had a small sail on its back, which could have been used to regulate temperature. Most dinosaurs are believed to have been cold-blooded. This means that as the air around them cooled, so did their bodies. They lacked the ability to generate their own heat. A sail would have allowed Dicraeosaurus and other sailed dinosaurs to gather in sunlight, and pass warmth to their bodies through their blood.