How to size rivets
The fetched Omni rivet size page sizes rivets from two quantities: the thickest plate and the total stack thickness. The minimum rivet diameter is three times the thickest plate thickness.
Dmin = 3 x tmax
L = S + A
Shank length allowance
Omni expresses the shank length as the total material thickness plus a head-formation allowance. The usual general-construction allowance is 1.5 times the rivet diameter, while the page also lists boiler and countersunk alternatives.
Grip range
The page's FAQ defines the approximate grip range as rivet length minus 1.5 times the rivet diameter. That gives a quick check on whether a chosen rivet will clamp the intended stack.