What size is a sheet of sandpaper?

There are several standards for coated abrasives, but by far the most commonly used are those of the Coated Abrasives Manufacturers Institute (CAMI) and the Federation of European Producers of Abrasives (FEPA). Worldwide, the FEPA system is more common. The two systems are not strictly comparable, because FEPA defines a grade by specifying a range of grain sizes, while CAMI specifies an average particle size. For example, by the FEPA standard for macro grits grade F180, no more than 3% by mass of the grit can have a particle size larger than 90 microns, and at least 94% must be larger than 53 microns. In F220 (a “microgrit”), no more than 3% can be larger than 75 microns, at least 50% must be in the range 50.0 to 56.0, and at least 94% must be larger than 45 microns. See the Standards section below for sources of detailed information.
In the abrasives industry, particle size is typically expressed in microns. But the CGPM, the controlling authority for SI, says microns should be called micrometers.