changing icon shadow size when zooming

I was wondering if there's a way to change the dimensions of a point's
shadow graphic after it's been rendered on the map? What I'm going for
is to use a circular shadow graphic to indicate radius from a given
point. When the user zooms in, I would increase the dimensions of the
png I'm using as the point shadow, and voila, the size of the radius
shown on the map would grow.

I thought this would be an easier solution than using one of the
third-party class libraries that depend on polylines and fills to draw
radius circles. But I can't seem to alter an icon's shadowSize after
it's been set. Is that just the way it is?