Measure the dimensions of a flat panel TV by carefully measuring the width and the height. The diagonal measurement of a TV is the measurement across the TV from one corner to the opposite corner.
The width is typically the longer side of a rectangular flat TV, and the height is typically the shorter side of the TV. The diagonal measurement across the TV is often listed in the TV descriptions at online retail sites.
In addition to measuring the height and width, the depth of the flat screen TV may be measured by measuring how thick the TV is from the front screen to the back of the TV. One might also consider measuring the dimensions of the base of the TV, if it has a base. The height of the base may be added to the overall height of the TV in some cases.