Monitor sizes aren't as straightforward as they seem. A 17-inch CRT monitor measures 17 inches diagonally across the glass face of its picture tube. However, the viewable area of the monitor--the diagonal measurement of the largest possible picture the screen can display--is never as large as the actual tube size. Thus a 14-inch monitor typically has a viewable area of 13.1 inches, a 15-inch monitor offers about 14.3 inches, and so forth.
There's one important exception: since the LCD (liquid-crystal display) panels used in notebooks can display images out to their very edges, their physical and viewable diagonal measurements are the same. Thus a 12.1-inch LCD panel in a high-end notebook offers very nearly the same viewable area as a 14-inch CRT monitor.
See also: CRT