Radius

The Radius slider defines the distance (in pixels) from an edge in which the contrast change occurs. You can see the difference in this diagram, which shows a simple low-res edge with three different Radius settings. The left image has Radius 0, so there’s no effect (despite a high Amount setting). The middle image has Radius 1px, and the right has Radius 3px. The larger the Radius, the further the sharpening is pushed away from the edge.