Every manufacturer will be different accept for the actual display area and all will be the same. So, a 23" monitor will have a display of 23", this is measured from the top left corner and measured to the bottom right corner, of the display area. Then, the chasis that houses the display area will differ as it will have the manufacturer's information and most likely set up controls buttons. And like mentioned above, some monitors can rotate for a different viewing.
I have an HP w2207h monitor (22"). So, from top left to bottom right it measures 22". The width of the viewing area(left to right) is 18 5/8", the height of the viewing area is 11 5/8". Around the outside of the viewing area is a 1" board along the right side, top, and left, the bottom is 1 1/4" to accomodate the menu button, volume up, volume down and a select buttom. So, complete dimensions for this monitor is 20 5/8" wide and 14" in height.
The depth of the monitor is 2 1/4". The monitor came with a funky stand that moves in every direction possible, on the fly, I can grab my monitor and turn it so it is 20 5/8" in height and 14" wide and the viewing area will automatically adjust(why would I want to do that? I don't know). The underside of the monitor has provision for VGA connector, HDMI connector and power. Then, along the side of the left side of the monitor has to USB ports. Oh, and on the top is a power button.