First look at what room the TV will call home. Whether the TV is in your relatively small bedroom or sitting at the far room of your large family room, location is a factor in determining the size of your TV. The comfort level in viewing the normal 4 to 3 ratio TV screen is somewhere between 3 and 6 times the size of your TV screen. This means that if you are looking at a 27 inch screen (by the way this means the diagonal measurement from one upper corner to the opposite, lower corner) you shouldn't be closer than five and a half feet or farther than eleven feet. If you want to go larger, for example, a 40 inch screen, make sure you have at least eight feet between you and the screen. If your den is small, don't get a TV that will have you moving your head to take in all the action. If your viewing room is more cavernous, your eyes will be straining at a too small screen and diminish your viewing fun.
If you are considering the wide-screen format for your new TV, the standards change somewhat. The wide-screen format TV is a 16 to 9 ratio and you can sit closer without discomfort, even as close as one and a half times the screen's diagonal measurement. So if you are looking at a 27 inch screen you can sit 3.5 to 7 feet away without quality depreciation.
If you are thinking the normal analog TV, count on 15 to 24 inches of TV tube depth. Generally, the larger the screen, the deeper the TV. Keep in mind these measurements as you think of where the TV will be placed. In a small bedroom, the TV might tend to take over the room if it is too large.