In order to focus an image, Focus Magic needs to know the amount by which the image is out of focus.  This is specified by the "Blur Width" and is the distance that an edge has spread (in pixels).  When you open an image, Focus Magic auto-detects the Blur Width, but it is advisable to experiment a little.

Examples of Different Blur Widths for an Edge

The "Blur Width" can also be thought of the diameter of the circle to which a single pixel spreads when the image goes out of focus.  Knowing this, a fairly accurate way to determine the blur width of an image is to look for point sources and measure their diameter.