The base radius of a shape used by the Simulate Rigid Bodies node for determining collisions. You can further modify the size used for collisions using CollisionScale, which allows you to scale by different values in each of the three axes.
The final size of a particle for collisions is the product of CollisionSize and CollisionScale. If CollisionSize is not defined, then is used, and if neither CollisionSize nor Size is defined then 0.001 is used. If CollisionScale is not defined, then Scale is used, and if neither is defined then (1, 1, 1) is used.