10 feet is a comfortable distance for a 42 inch for me.
found this article : The website can give you an indication.http://www.dtvcity.com/lcdtv/lcdscreensize.html
Selecting the correct LCD TV screen size.
Sitting the right distance from your LCD TV is an important part of optimizing the viewing experience. Sitting too far away from a small- or medium-sized screen definitely diminishes the overall impact. At the same time, sitting too close to a large screen is also less than ideal. A good indication that you're sitting too close to a screen is if you find that you're distracted by being aware of the screen's "structure" — those tiny "dots" (pixels), and for CRT-based TVs, horizontal scan lines. If you're not sure how far away you should sit for optimum viewing, consult our suggested viewing distance chart.
Q: What is the optimum viewing distance for different screen sizes?
A: There are differing opinions on the best way to determine optimum TV viewing distance. Generally, you calculate viewing distance by multiplying one of the screen's dimensions (height, width, or diagonal) by some fixed number. The rule-of-thumb generalizations for standard analog TVs with 4:3 aspect ratio no longer hold true for newer HDTV sets and HDTV-ready TVs. These high-resolution models include upconversion circuitry and wider 16:9 aspect ratio screens. High-resolution displays reduce the visibility of scan lines, so you can sit closer without noticing them.
The calculator above can give you recommendations for screen size and/or viewing distance. The top part of the calculator will give you an optimal viewing distance based on the screen size you enter in the left-hand column. The lower part of the calculator should be used if you know where you want to put your television but are unsure of what screen size to choose. Measure the distance from the area where you will place the TV to your normal viewing area, enter the amount in feet in the left-hand column and the calculator will display an optimal screensize in the right-hand column.
Q: What about viewing height? Does that matter?
A: For optimum viewing, your eyes should be level with the middle of the screen when you're seated in your normal viewing position. This is especially important for a "big-screen" rear-projection TV since its picture will look its brightest at this level. The cabinets of larger rear-projection TVs are designed to position the screen at the optimum height.