https://player.vimeo.com/video/750289068?h=a20556a56a&badge=0&autopause=0&player_id=0&app_id=58479
Set at Particle Radius, by default.
The other options are: Particle Scale and Particle Mass.
In this setting, the modifier will affect the radius (not the scale) of the particles emitted.
Animation demonstrating Parameter to Change set to Particle Radius option.
The modifier will affect the particle scale value and will open up additional parameters.