My vote is 27" whenever possible.
Why? Simple. Anything smaller than 19" is too small (unless it's a bartop.) So, unless you have dedicated vertical and horizontal cabs, the very important vertical games are going to be too small on a 19" horizontally oriented monitor.
On a 27" horizontally oriented monitor, the vertical games viewed on it are almost exactly the same size as a 19" vertically oriented monitor. This gives you a REALLY big screen for horizontal games (which I actually prefer.) But if it's too big for you, shrink it in software by setting the screen res to something higher than the game's native res and turn off scaling.
RandyT