To measure a TV, place a tape measure or yardstick diagonally across the screen and measure from corner to corner. Measure only the screen, not the TV's case, unless trying to find dimensions to meet overall space requirements.
Regular televisions and wide-screen TVs have different aspect ratios, though the sizes remain the same. For example, a 26-inch widescreen TV is a 26-inch with a 16:9 aspect ratio and a regular 26-inch TV is a 26-inch TV with a 4:3 aspect ratio.
While the screen's measurement defines the size of the television, if measuring the TV for a stand or to put in an entertainment center, then the outer measurement can be important. The measurement is still diagonal, across the unit from the an upper outer corner to a lower outer corner.