If you had read the Collisions command you would have seen this:
In the case of a collision detection method of 1 being selected (ellipsoid-to-ellipsoid), then the destination entities concerned will need to have an EntityRadius assigned to them too. In the case of method being 2 being selected (ellipsoid-to-box), then the destination entities will need to have an EntityBox assigned to them. Method No.2 (ellipsoid-to-polygon) requires nothing to be assigned to the destination entities.
So, the answer to that question is if you use EntityBox, you do not need to assign an EntityRadius (unless you want to).