Size and Weight:
Ornithomimus is the type of dinosaur that movie and TV directors love to show stampeding in herds across vast prehistoric plains: this theropod may have been able to run as fast as 30 miles per hour, and it may also have been covered in feathers. Unlike an ostrich, though, which it superficially resembled, Ornithomimus was definitely a land-bound dinosaur, and not a bird. It was closely related to other ornithomimid ("bird mimic") dinosaurs like Dromiceiomimus and Gallimimus, which may have been even faster.
Like some other human-sized theropods of the late Cretaceous period--most notably Troodon--Ornithomimus was also distinguished by its fairly large brain. While it's unlikely that this "bird mimic" was as smart as a modern ostrich (and that wouldn't be saying much), this is a tantalizing clue that carnivorous dinosaurs were on the verge of evolving bigger brains before they went extinct.