I have found Laser beam pathway with reflective mirrors - and I would like to run the example there: 
Unfortunately, this is an example from some years ago, and I use Blender 2.92; so when I click Bake, and then click Play as recommended, I get this:
I guess those massive balls in the screenshot should be small particles? In any case, they seem to be not selectable with the mouse...
So I found how can I change the size of particles? - but that question is even older, and I have no "size" under Physics.
So, how can I control the size of the particles in that example in Blender 2.92?



