I have a 27" and love it, but it's mounted vertically. I love how "in your face" it is for shooters. Make sure you can get the distance you need because if you have to look around too much during play, it gets tiring. I know that sounds silly, but your neck and shoulders will start to fatigue as well as your eyes. For me, looking back and forth is more bothersome than up and down, so its important to get the distance right with a large horizontal monitor.
I actually think 25" is the perfect size, but not much of an option anymore.