Make sure the screen is TFT - I'm not sure if the (very) inferior DSTN screens still exist, but they may do on some very cheap laptops. Resolution should be 1024x768.
Apart from that, screen size is a matter of taste, really - 12.1" is a minimum IMHO, and the larger the better. However, larger screens cost more and aren't as light (if that matters). Think about how much time you are going to spend using it, but remember that you can always buy a 17"/19" CRT to plug the laptop into if you're working in one place a lot.