Field of view.
More than about 19" is too wide for reading, and especially reading glasses.
If you want to simulate a real desktop then you need about 1700 pixels high at 200 dpi, or about 2500 pixels high if 300 dpi.
About 133dpi is a minimum if you rely on sub pixel addressing at edges of fonts, but 200 dpi or higher with subpixel addressing on text off is better.