I'm no big expert on this, but think of grit as the size of screen the substrate will filter through. Depending upon the specifics of your tank, it seems that the general recommendation is particles between 1 and 3 millimeters.
Ah! Here, I found it: Grit - The nominal size of abrasive particles corresponding to the number of openings per inch in a screen through which the particles can just pass. -- IOW, if one were to choose 20 bits from 20 grit substrate and line them up, they would measure about an inch. 50 grit particles are smaller than 20 grit.
Hope that helps.