the radius IS the "size" of the camera, the camera is a virtual camera, not a in game object, it is invisible and collision is set by wrapping it in an invisible sphere you can alter the size of to enable it to go into tight places etc, if you make the collision radius (the invisible sphere) too small you will find you can walk up to walls and they disapear because the cameras default cliiping range is too high and you get close enough to clip the objects, so just use
set camera range 0.001,3000
that gives you the normal distance camera clipping range but objects have to get less than .001 world units away before they get clipped, if you make the collision radius smaller than that then you will have to reduce the value further to prevent objects getting clipped, hope thats some help, cheers.
Mentor.