The particle size in deposition requires the definition of the manufacturer of the measuring equipment. Unlike working with polysirene equivalent dust to calibrate an airborne counter,  the deposition counting and sizing is done under a automatic optical microscope scanning the disk.  The optical system is calibrated by means of optomechanical standards.  Around each particle the smalles fitting rectangal is drawn. The longest side of this rectangal is the size of the particle published.  It seems to be  good system.  Flat particles such as skin particles appear in deposition somewhat bigger than airborne.  Skin particles can often be recognized in the images.  Download a short explanation here with examples here