Sharp provides the following information.

You can sit closer to an HDTV than you can to a standard TV without noticing the pixels (dots of color in the screen).  For standard TVs, the normal comfortable viewing range was 3 to 6 times the screen size in inches.  For HDTVs, the normal comfortable viewing range is 1.5-3 times the screen size in inches.

Given that information, here are their recommendations for screen sizes

If your viewing distance is 4 feet, then you want a screen that is 19″ to 32″ in size.

If your viewing distance is 6 feet, then you want a screen that is 26″ to 46″ in size.

If your viewing distance is 8 feet, then you want a screen that is 32″ to 63″ in size.

If your viewing distance is 10 feet, then you want a screen that is 40″ to 80″ in size.

If your viewing distance is 12 feet, then you want a screen that is 46″ to 96″ in size.

If your viewing distance is 14 feet, then you want a screen that is 52″ to 112″ in size.

These sizes are calculated by the following equations:
Minimum size = viewing distance/3
Maximum size = viewing distance/1.5