When people think of dinosaurs they think big But there were small dinosaurs to the smallest being about the size of a chicken what is the name of this dinosaur?

Actually, the smallest known dinosaurs were even smaller than chickens. Epidexipteryx weighed only 0.36 lb, and was less than 10 inches long. You are probably referring to Compsognathus, which was actually somewhat larger than a chicken (the chicken sized estimate was based on a fossil of a juvenile before it was realized that the individual was not fully grown).