Size and Weight:
One of the most primitive of all theropod dinosaurs, there's some dispute about whether Herrerasaurus was even technically a dinosaur at all--that is, this carnivore may have predated the primordial split between ornithischian ("bird-hipped") and saurishcian ("lizard-hipped") dinosaurs, which could conceivably have made it a very advanced archosaur. Whatever the case, it's clear from Herrerasaurus' predatory arsenal--including sharp teeth, three-fingered hands, and a bipedal gait--that it was an active and very dangerous predator, even making allowances for its small size.
According to a recent analysis, it's a good bet that the earliest dinosaurs evolved in South America during the middle Triassic period, when Herrerasaurus lived, and then gradually spread to other parts of the globe. In fact, the fossil beds where Herrerasaurus were found later yielded another famous proto-dinosaur, Eoraptor. (In case you were wondering, the reason this dinosaur is named Herrerasaurus is because its fossils were discovered by an Andean farmer named Victorino Herrera, in 1959.)