Red blood cells are about 6 microns across - that's 6 thousandths of a millimetre - but they are very easy to see.
Sperm are smaller, at about 1 micron across, but longer (length about 25 microns). However, they can easily be visualised under a light microscope. I've looked at mine !
To put that in perspective, the diameter of a capillary is about 4 microns across (half the width of a red cell) and a bacterium is about 1 micron in diameter. A human hair measures about 25 microns across.
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx