The viewable area is not the size of the cathode ray tube but the actual area of the screen that is not blocked by the monitors plastic casing. Manufactures derive viewable area numbers by measuring diagonally from corner to corner. The image on most CRT monitors is composed of millions of tiny colored phosphor dots arrayed in horizontal rows. The monitors resolution is determined by the number of dots spanning one horizontal line multiplied by the number of horizontal lines. The closer the dots are to one another, the sharper the image.
Dot pitch is the distance between two like-colored phosphor dots.
Buy a monitor with a dot pitch of .25 or below and don't consider a monitor with a dot pitch in excess of .28. Without a fast refresh rate your monitor will flicker. Vertical refresh rate is a measure of how many times per second the screen gets redrawn up and down. A monitor with a refresh rate of 75Hz gets redrawn 75 times per second which is standard for most people. Choose a higher refresh rate if you are doing lots of games.