Not unimaginably small

Typical human cells are about 10µm in length. This is roughly 1,000 times smaller than the last joint in your finger.  A 1,000-fold difference in size is not difficult to visualise: a grain of rice is about 1,000 times smaller in length than the room you are sitting in. Imagine your room filled with grains of rice. That will give you an idea of the billion or so cells that make up your finger tip. 

Another 1,000 times reduction takes us to the world of molecules...An average protein, taken from any cell, contains about 5,000 atoms and is about one-thousandth the length of a typical cell, or about one-millionth the width of your fingertip. Again, to get of the idea of these sizes, think of a room filled with rice grains. This will give an idea of the size of the proteins that are packed into each of your cells.

— from The Machinery of Life by David Goodsell (2009)

Image: macrophage and bacterium by David S. Goodsell, the Scripps Research Institute