There are a many factors which can be considered in determining the optimum viewing distance including pixel size and limits of the human vision system. The following tabulation gives optimum viewing distance recommendations for both 1080p and 720p resolutions for various HDTV screen sizes.

To see the most of the detail delivered by a high definition HDTV, one should sit as close as is practicable. Sitting too close though allows you to see the pixel structure and destroys image quality. Because the pixel size for a 1080p display is smaller than that for a 720p of the same size, one can sit closer as seen in the tabulated recommendations.

It’s worth noting that the immersive effect of a HDTV screen filling a wide angle of your field of view is lost if you sit over 3 times the diagonal width of the screen from it. If you sit in front of a 1080p HDTV at the optimum distance for 720p HDTV you may as well have saved some money and bought a 720p model as you won’t notice any difference.