I use a 21" monitor at home, and while it's huge, I don't think it's too close. I certainly will never go back to a smaller monitor...
Also, I don't believe that the size of the monitor has anything to do with the quality of the image. If you buy a 'good' 21" monitor, the quality is gonna be the same as a 'good' 19" monitor. For quality, I would be more concerned with the screen being flat or near flat, the dot pitch, and the refresh rates the monitor can support at high resolutions. If you read the forums more on monitors, you'll get an idea of what manufacturers people prefer here...