Microfossils are typically measured in units of micrometers (microns).
A micrometer is symbolized using the Greek letter mu ‘μ’, and ‘m’ (μm). One μm = 1 millionth of a meter or 1/1000th of a mm
In our lab we typically work with microfossils in the 5-50 μm range, but phytoliths can be 100 μm or more in size. Find out what they are made of here.