In size their cells are commonly about 0.001 mm. (1 micromillimetre or 1 µ) in diameter, and from two to five times that length, but smaller ones and a few larger ones are known.    From Wordnik.com.  [Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 2 "Baconthorpe" to "Bankruptcy"] Reference

A micromillimetre (μ) is 1/1000 of a millimetre or 1/25000 of an inch.    From Wordnik.com.  [V. Angiology. 2. The Blood] Reference