How big is a micro-organism? Small, little, very tiny, too small to see, microscopic-but how big is tiny or too small to see? When we give the size of a microbe, we use the metric system so we do not say one-thirty second of an inch, 1/10,000 of a foot, or other sizes in units like inches, feet, or yards. Measuring microbes uses a unit called the micron. A micron is 1/1000 of a millimeter (mm.) or we can say there are 1000 microns in a millimeter. Could you see an organisms 1 mm. in size? For most, probably all, of us the answer is a definite "yes."