The size of particulates is generally described by the "aerodynamic" diameter of the particles. Common size designations are PM 10 = particles that are 10 micrometers (um) and smaller, PM 4 = particles 4 um and smaller (these are often referred to as "respirable"), and PM 2.5 = particles 2.5 um and smaller (these are often called "fines").
Image: http://www.sbcapcd.org
The size of the particle is the principle factor determining where in the lung the particle is deposited, as seen in the figure below:
Image adapted from http://www.atmosphere.mpg.de
Sand used for water purifying is in the 500-1000 um (1 mm) diameter range (http://ppoa.org/?page_id=399, http://en.wikipedia.org/wiki/Sand_filter). The smaller "cementing" particles in intact sandstone of most concern to human health are in the 2.5 and lower range (PM 2.5, or "fines"), about 300 times smaller.
