The most important are screen size and dot pitch. The screen size is usually stated in inches and the dot pitch will be a number like .28 or .27. The larger the screen size and the smaller the dot pitch, the better. I never recomend any monitor smaller than 15 inches. I much prefer a 17 inch monitor, especially at higher resolutions.
Screen resolution is a measure of how much information is displayed on the screen at one time.
The smallest resolution used normally is 640 by 400, the old VGA default. Resolutions supported by video cards and monitors now go much higher. Many people run thier computers at resolutions of 600 by 800 or 1024 by 768. When you run at a higher resolution, text and graphics are smaller on the screen. At higher resolutions, a larger screen size on the monitor can make all the difference.