There's a huge number of different species of wasp, which vary greatly in size. The smallest wasps are the smallest known insects; they're literally smaller than a millimeter. With many of them, you wouldn't know they were wasps if you didn't have a microscope.

In addition to pollinating, many wasps either eat or parasitize other insects. Yellowjackets will hunt horse flies, and there's assorted wasps that are sold to farmers to control various pests..