Here's what I could find for you.
From Adobe:
Radius Controls the size of the local neighborhood around each pixel. Neighboring pixels are used to determine whether a pixel is in the shadows or highlights. Moving the slider to the left specifies a smaller area, and moving it to the right specifies a larger area. The optimum local neighborhood size depends on the image. It’s best to experiment with the adjustment. If the radius is too large, the adjustment tends to brighten (or darken) the whole image rather than brightening the subject only. It’s best to set the radius to roughly the size of the subjects of interest in the image. Experiment with different Radius settings to obtain the best balance between subject contrast and differential brightening (or darkening) of the subject compared to the background.
Remember that this command uses the luminance of surrounding pixels, called local neighborhood, for adjusting the shadows and highlights. The best local neighborhood size depends on the particular image, so play with this slider and view the results. If the Radius is too small, your main subject may lack contrast. Conversely, if it's too large, your background may be overly bright or dark. Adobe recommends setting the radius to approximately half the size of the main subject in your image. So if your subject takes up roughly 600 pixels, then set your radius to 300 pixels. Choose Show Rulers on the View menu and set your Units to pixels in your Preferences.
I hope this helps,
Steve
