There are actually two cells which are considered to the be smallest. In the human body it is the sperm cell. A sperm cell looks like a tadpole (baby frog). The size of a sperm cell is measure in microns which equate to about 25400 per inch. The sperm cell is about 40 microns. The second cell is actually bacteria (mycoplasma) and it is a whooping 0.0001 mm. I bet you never saw that one coming.