Size:
Bigger is better? Don't go to big! Use a desktop size monitor, with a desk/table.
Response Time:
Low response time reduces ghosting and smearing. It will look sharper and clearer.
2ms or less is fantastic.
Refresh Rate:
This is how often the screen displays an image, and is one of the most important specs to look for. Higher refresh rates will make the game look much smoother and will also prevent screen tearing (so we don't have to use vsync which causes input lag.. YAY!!). Typically refresh rates are 60-75 Hz. 120 Hz or higher is a good refresh rate to look for.
Resolutions:
16:9 resolutions give you a larger field of view than 4:3. Try and get a 16:9 screen.
Largest resolutions are preferred.