Great Question!

Generally, you want to multiply the screen sized measured diagonally, by 1-1/2 and 2-1/2.  So for a 42" screen for example, the viewing distance should be between 5.25-8.75 feet.  However the ideal distance may vary depending on several factors, including signal quality and HDTV resolution.

Take a look at our own Dell Home Theater Learning Center for assistance or check out CNET's Ultimate HDTV Buying Guide for additional information on viewing distances.