– The bigger this value the bigger the radius of the particles.

– You can see how particles with different sizes interact with the green plane set as deflector.

– Remember: all particles seem to have the same size but it’s only because radius is not visualized!

[download .blend]

– Note: at this moment, with Blender 2.8 Beta git.80ffa73b3f87 I cannot render the Halo material, so I rendered this video using Blender 2.79b