That Is what he means by Computer Screen
A screen is a part of a monitor - i.e. the bit that shows the display. And I doubt mervyn797 needs you to patronise him either, btw. :3
With VDUs, as far as I know, the best option really is to just find a compromise between size and price. The lowest you'll want is 13.3", which corresponds to a maximum resolution of 1024x768 (1280x800 if it's widescreen). In practice, 15" or 17" are probably what you'll want, so search around for the cheapest options of those size among the respected brands. :)