Monitor size is measured by the distance from one corner of the screen's viewable area to the diagonally opposite corner. With CRTs, the number is generally about an inch greater than the actual area. Starting in the late 1990s, the real number is included in the specs. For example, you might see a 17" monitor state 16.1" viewable.
With flat panel monitors, the screen size specification is actually its real size. What a concept! See
CRT,
flat panel display and
screen resolution.