Dinosaurs ranged in size from the size of a chicken to well over 100 feet long. Most dinosaurs were somewhere in the middle. The modern-day blue whale is the largest animal that ever lived - it is larger than any dinosaur was. No one knows why some of the dinosaurs (and some other Mesozoic Era reptiles) got so huge and none do today. This is one of the most interesting unanswered questions in paleontology. There are a lot of theories, but some of them are pretty bizarre; none are generally accepted

LONGEST
The longest dinosaurs were sauropods; they were gigantic, slow-moving, tiny-headed, cow-like plant-eaters from the late Jurassic and the Cretaceous period. They had very long necks which were useful for reaching wide (and tall) swatches of vegetation. The lengthy neck was counterbalanced by a massive tail. These sauropods are the largest land animals ever discovered:


Supersaurus - 134 feet long (41 m)
Argentinosaurus - 115-130 feet long (35-40 m); 80-100 metric tons
Seismosaurus ("Earth-shaking lizard") - 120+ feet long (37 m); +80 tons
Ultrasauros - 100+ feet long (30 m), +80 tons
Diplodocus - grew up to 90 feet long (28 m).
Brachiosaurus - about 85 feet long (26 m), 40 feet tall, and weighed 70-80 tons.

The biggest carnivores were theropods from the Cretaceous period:
Giganotosaurus

Giganotosaurus carolinii - found in Patagonia, Argentina. 47 feet long (14 m), 8 tons in weight, and 12 feet tall (4 m).
Tyrannosaurus rex - found in North America - 40-50 feet long (12-15 m), 6 tons in weight.
Carcharodontosaurus saharicus - (meaning shark-toothed lizard) found in in Morocco, Africa, by Paul Sereno. It was probably about 45 feet long and had 5 inch long teeth. Its skull (5 feet 4 inches) is larger than T. rex's, but it had a tiny brain cavity (half the size of T. rex's).

TALLEST
The tallest dinosaurs were brachiosaurid sauropods; they had front legs that were longer than their back legs and had a giraffe-like stance. They were gigantic, slow moving, tiny-headed, cow-like plant-eaters from the late Jurassic and the Cretaceous period. They had very long necks which were held upright and were useful for reaching wide (and tall) swatches of vegetation. The lengthy neck was counterbalanced by a massive tail. These sauropods are among the largest land animals ever discovered:
Brachiosaurus

Sauroposeidon, a newly-found brachiosaurid from Oklahoma, +60 tons, 60 feet tall.
Ultrasauros - 100+ feet long (30 m), +80 tons, +40 ft tall
Brachiosaurus - about 85 feet long (26 m), 40 feet tall, and weighed 70-80 tons.

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