it's all opinion bud. think of it as resolution. the higher the resolution, the better the accuracy. unless of course the resolution is huge and your gun has horrible accuracy.
in a perfect world, bigger would be better and i wouldn't go for anything less than 19. but that's alright since i've got a 27.
also think of how far back you'll be from the screen. the larger it is, the farther back you can stand. i wouldn't play on a 21 inch screen just because it would be like shooting fish in a barrel because you'd have to be so close. but again, it's all opinion.
if it were me, i'd be more worried about a budget rather than how good it will play.
21 = free
something else = 250 (tv or large pc monitor)
27 = 500 (multisync arcade monitor)