Here's why:

* Centimeters (cm) are too large. An eyelash is much smaller than a centimeter.

* Millimeters (mm) are a good size. They offer a level of precision suitable for measuring something as small as an eyelash.

* Micrometers (µm) are even smaller and might be necessary for extremely fine measurements, but millimeters are usually sufficient.

Of course, if you're looking for extreme precision, you could always use a tool like a microscope and measure in micrometers.