Well, I just tried this with a cat toy ball, and a lamp here at my desk, and from the looks of it, it seems that the size of the shadow doesn't scale out in X and Z more that it really just blurs out and decreases in opacity.
So, I think your best be for this, might be to render out a separate shadow pass, (render layers option), and blurr/scale it to taste in post.
lemme know if this helped at all
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS