In ArcGIS 10, users can define the neighborhood in the Focal statistics tool. The moving window shape can be set to any of the following: Wedge, annulus, circle, rectangle, irregular, and weight. For example purposes, let's assume we want to calculate the mean tree canopy area from a binary classified canopy raster (i.e. 0 = no canopy and 1 = canopy present) at 1 m spatial resolution. Given this example:

Are there any rules of thumb to determine which moving window shape should be used?

Once you determine the moving window shape, is there way to quantitatively determine the optimal neighborhood settings, or is this entirely subjective?