Screen size refers to the size of the monitor display screen. It is one of the primary criteria on which monitor purchases are made. This size is measured in inches and is measured diagonally, i.e. from one corner to the diagonally opposite corner. Keep in mind that a monitors actual screen size is not the exact size as the viewable image size. To know the exact size of the display screen, remove another inch or so from the published screen size. The most common sizes available now are 14, 15, 17, 19, and 21 inches, although many manufacturers are phasing out 14.