The final size of a particle for collisions is the product of CollisionSize and CollisionScale. If CollisionSize is not defined, then Size 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.
For example, setting this value allows you to use a slightly larger size for collisions than for rendering and display, reducing interpenetration. You can also scale by different values in different axes for oblong collision shapes that cannot be defined by Size or CollisionSize alone.