Spinosaurus aegypticus may have been the largest terrestrial carnivore to ever walk the Earth. While its remains (sketched above by Stromer when he described the beast in 1915) were destroyed during WWII, a new snout specimen described by Dal Sasso et al. (2005) estimated the total body length may have been between 16 and 18 meters, making it significantly larger than Giganotosaurus, another monster theropod on this list. Other members of the Spinosauridae, like Baryonyx and Irritator, reached respectable lengths (30 feet, 36 feet) but they were dwarfed by their ultimate decendant.
Giganotosaurus was the largest member of the Carcharodontosauridae, an enormous Gondwanna branch of the Allosauroidea. Its body was built like a typical allosaur, but its head was long, tall, and built for killing sauropods. This 50-foot monster was the first dinosaur unearthed to beat out Tyrannosaurus rex for "biggest carnivorous dinosaur," and rightly deserves the title. While the tyrant lizard king is a more compact killing machine, Giganotosaurus was leaner and longer, focusing more on speed than strength. While this gigantic lizard couldn't crush bone, it would have delivered a fatal tearing bite to any prey item foolish enough to stand its ground.
Discovered by Barnum Brown in 1902, Tyrannosaurus rex was, for a very longn time, the largest carnivorous dinosaur known. While it no longer retains that title, it does get to keep another: strongest carnivorous dinosaur known. Tyrannosaurus rex could shatter bone with its bulldog grip, which left the arms with nothing to do, apparently. Although huge, Tyrannosaurus was compact for its size, with a short midsection, thick but short neck, and rounded muzzle. Compare Tyrannosaurus' skull to Giganotosaurus', and the differences are readily apparent. Tyrannosaurus seems to have gone through several growth phases, during which its diet and mode of prey capture may have changed. Thomas Holtz has wondered if this single species dominated the carnivore scene at the end of the Mesozoic. This would have made Tyrannosaurus rex the most powerful and most flexible carnivorous dinosaur known!
In a family of lightweight, midsized theropods, Deinocheirus really stuck out in a crowd. The Ornithomimidae is usually characterized by Struthimimus, Gallimimus, and Ornithomimus. The only known remains of their gigantic cousin--a pair of fearsome-looking arms, demonstrate that one of their ranks towered over the rest. If you scale the arms of Deinocheirus to one of its smaller brethren, you get an monsterous ostrich mimic that would have rivaled Tyrannosaurus in size. It could be that Deinocheirus' arms were out-of-proportion to the rest of its body, or that it wasn't an ornithomimosaur at all, but until more complete remains are found, Deinocheirus will remain the largest of the ostrich mimics.
While many theropods became very large in order to hunt larger game, one family increased in size due to a basic change in diet. The Therizinosauridae is a bizarre group of maniraptor theropods which switched from meat to vegetation during the Early Cretaceous. Very quickly, their anatomy changed to accomodate this change. In addition to an enlarged gut, retrovated pubis, hypertrophied pedal digit I, widened pelvis, and renovated pectoral girdle, therizinosaurs also began growing. By the end of the Cretaceous, Therizinosaurus was 40 feet long and may have converged to some degree with the lifestyle of sauropods, which had since disappeared from Laurasia. Not much of Therizinosaurus' skeleton is known, but it is most famous for its enormous, scythe-like manual claws. Whether it used the claws for defense, digging, or reaching tall branches, we may never know.
The smallest "giant" on our list may also be the deadliest. Utahraptor was a massive dromaeosaur, and may have reached between 18 and 25 feet long. While virtually none of its bones are known (a short caudal series, maxilla, and giant claws), scaling the beast to its close cousin Deinonychus results in a raptor that's almost twice as large! Utahraptor lived alongside Deinonychus during the Early Cretaceous, and neither animal had to compete with tyrannosaurs, which did not impede on their territory until the Late Cretaceous. It may have hunted resident ankylosaurs and ornithopods, as well as North America's remaining brachiosaur, Cedarosaurus.