The optimum viewing distance for NTSC sets is 8 times the screen height. If you sit closer, the picture will be blurry and you may feel some eyestrain. For a HDTV set displaying 1920x1080 pixels, the optimum viewing is 3 times the screen height. The optimum for 1280x720 pixels is 4.5 times the screen height. Note that for people who always sit at the optimum distance, “high definition” is actually a misnomer. “Giant picture” is what these sets really are. HDTV is more like a movie theater experience than it is the traditional TV experience. “Home theater” is now the truth.
Note also that just because your HDTV is 3 times bigger than your old NTSC TV doesn’t mean that you can sit 3 times as many people in front of it. In fact you can’t fit any more people in front of it, at least if you want the full theater experience. Note also a built in annoyance: Every time the broadcaster changes resolution you have to move your chair. No one actually does this. They just suffer the mismatch. The only solution is to wait patiently for the day when all broadcasts are hi-def. Until then, room layout is an unsolvable problem.
(Many NTSC viewers sit in the range of 5 to 7 times the height, feeling that the advantage of a big screen is worth a slight blur. The scan lines become visible at 4 times the screen height. If you can see the lines, you are sitting way too close.)