Size of the screen is not the only concern. Some monitors display colors more "truely" than others. Generally a cheap monitor will look too blue or too washed out or will be slightly brighter around the edges than in the center, or something equally nasty. All of those things can screw you up when you're trying to draw something.

For example I have an Apple 20 inch widescreen monitor I paid $700 for last year, and an extremely cheap 17" 'MAG' brand monitor I got on sale for $125. If I take a drawing and drag it from one monitor to the other, the difference in color and sharpness is shocking.