Screen Size and Seating Distance
The site I use for that purpose is here. When you open this page, scroll down to the Viewing Distance Calculator section, enter your seating distance in feet, and click on the 16:9 button to specify the screen shape. Then, click on the Calculate button and look at the Screen Size section. For your seating distance of 7 feet, you want a 16:9 screen that measures about 52 inches diagonally to achieve the minimum SMPTE-recommended viewing angle of 30 degrees or about 63 inches for the THX-recommended viewing angle of 36 degrees.
The optimum screen size for a given seating distance is much larger than most people think. Most folks sit farther from the screen than you do—typically 10 feet or more—and they buy TVs with much smaller screens than they should—often 42 to 50 inches, which is smaller than you should get even at 7 feet. So my advice to all TV shoppers is to buy a larger screen than they are probably considering and to use the site linked above to determine the best screen size for their seating distance.
You can also use this calculator if you already have a TV and want to know the optimum seating distance for that screen size. For example, let's say you have a 50-inch, 16:9 TV. Enter that info into the corresponding fields and click on the Calculate button. As you can see above, the maximum SMPTE-recommended seating distance is 6.8 feet, which yields a viewing angle of 30 degrees. The maximum THX-recommended distance is 7.9 feet for a viewing angle of 26 degrees, and the optimum THX-recommended distance is 5.6 feet for a viewing angle of 36 degrees.
Interestingly, this calculator also provides the maximum viewing distance based on visual acuity—that is, the distance beyond which you cannot see all the detail in the picture because of the limitations of the human visual system. In the case of a 50-inch, 16:9, 1920x1080 display, that distance is 6.5 feet.
If you have an A/V question, please send it to email@example.com.