We’ll never know for certain, but the smallest dinosaur yet discovered is the Microraptor, a small bird-like theropod which is thought to have been about 18-24 inches long. Prior to this, the Compsognathus, another theropod was thought to have been the smallest dinosaur.

The Compsognathus was about the size of a modern chicken and weighed 5-7 pounds. Other small dinosaurs discovered so far are the: Saltopus (an insectivore about 24 inches long), Lesothosaurus (a plant-eater from Africa), and the Wannanosaurus (another plant-eater about 39 inches long).

Source: Enchanted Learning, dinosaur questions.