The word particle is general in meaning. Whether an object can be considered particles depends on the scale of the context; if an object's own size is small or negligible. For example: Galaxies are so large that stars can be considered particles relative to them. Grains of sand on a beach can be considered particles because the size of one grain of sand is negligible compared to the beach.

The size of a particle is usually described in microns as a replacement for the traditional thou or mil, each of which represent a thousandth of an inch. There are 25,400 microns in one inch. The eye can see particles to about 40 microns. Below are just a few examples:
Particle Particle Size (microns)
Pollens 10-1000
Human Hair 60 -600
Cement Dust 3-100
Mold Spores 10-30
Textile Dust 6-20
Spores 3-40
Coal Dust 1-100
Auto Emission 1-150
Bacteria 03-60
Beach Sand 10-10000
Dust Mites 1-3