Televisions are measured according to their screen size, diagonally across the screen. To measure a TV, take a tape measure and use the bottom left-hand corner as a starting point. Stretch the measuring tape upward across the screen, diagonally to the top right-hand corner. The length is the screen's measurement.
The reason televisions are measured diagonally is because of non-standardization of TV construction in the industry. For instance, the same-sized screen is often housed in different-sized frames. Televisions may also have differing aspect ratios, so that the ratio of the height to width of the screen differs from one television to another. This can result in screens of the same diagonal length differing in the widths and heights of the screens.