Does the size of the particle matter in their motion equations?
@BellaJ I believe their mass matters, but their size is usually ignored except for determining collisions. Though size technically affects drag/wind resistance, the effect of that is usually pretty negligible.
@BellaJ Particle systems are typically "pseudo-physical," so the size of the particle can take on many different meanings, and the visual size may not correlate in any obvious way to the mass. Just depends on what you're trying to model---and how accurate you want this to be! But yes, if you are trying to simulate something that really behaves like a bunch of physical particles (say, a bunch of bouncy balls flying around a city!) then you may want to make mass proportional to size.
And, as @silentQ mentions, this size may affect other things like drag or collisions.