Determining flat screen size follows a standard formula, regardless of whether you are measure a computer screen, a CRT TV, or a widescreen TV. Once you have that measurement, you can determine what screen size is optimal for your viewing habits. Note that this is of particular interest when selecting a TV, since a screen that's too small will be difficult to see while a screen that's too large reduces the perceived quality of the image.
Use a tape measure or yardstick, measure in inches from the lower left corner to the upper right corner of the screen itself. Don't include the plastic paneling that frames the screen in your measurement. The sides of the viewable area will not necessarily display at all times, if the aspect ratio doesn't match the TV's widescreen aspect ratio. Nevertheless this area is included in the diagonal measurement.
Begin determining the optimal TV screen size by measuring the distance you'll be sitting from the TV.
Divide this distance by three to get the upper limit of TV measurement. For example, if you're eight feet from the TV, that's 96 inches. So take 32 inches as the upper limit for the TV size.
You may object to measuring a flat screen this way, since its flatter aspect ratio (larger ratio of width to height) gives a greater measurement than a CRT screen of the same viewing height. Some people even say that a 19-inch CRT screen looks bigger than a 19-inch flat screen. However, measuring diagonally from corner to corner is the standard method of measurement. Therefore, be careful when comparing CRT measurements to flat-screen measurement to avoid an apples-to-oranges comparison.