I work on blood cells which are quite tiny about 10 microns wide. One thousand microns make 1 millimeter (that’s the littlest marking on your ruler) so you could fit 100 blood cells into 1 millimeter! pretty tiny eh?
The smallest cell I have researched has been around 30 microns (from a colon cancer) so bigger than a blood cell. However, single cell organisms called mycoplasm are contaminants that can grow in my cancer cell cultures and these are really really small only 0.1 microns which makes them very difficult to detect!