What was the smallest dinosaur that ever lived?

The bee hummingbird (Mellisuga helenae) is "the smallest dinosaur that ever lived". Of course your question only applies to non-avian dinosaurs.

For over a century the smallest known dinosaur was the 3 foot Compsognathus, otfen compared to chickens in size. The discoveries in China for the last 20 years have revealed many smaller dinosaurs. Currently the smallest known dinosaur is Epidexipteyx, at only 1 foot long.

The smallest dinosaur that ever lived on earth was "westlothiana," it is almost the same size as a fully grown human's toe!