-Number of inches. 15 should do the trick, 17 preferred.
-Good response time, preferred =< 20ms
-Good contrast, preferred >= 1:450
-Good brightness, preferred >= 350 cd/m2
The Benq FP591 is the cheapest monitor that complies to that characteristics, in my country (E279). But it my be different in other countries.
For other solutions which comply to this characteristics, look at
The site is Dutch, but at the green bottom table you'll find some other LCD screens that comply, and you can change the charasteristics the named screens should comply to. The values are as I said here.