I used to have a "24 flat screen, and it was nice for the time. When I got some good money, I went and bought a "42 flat screen and it really makes a great difference, especially playing from the recliner.
I belong to the blank generation - Richard Hell http://www.last.fm/user/akuma634\
My personal preference is around 23-24", for the same reasons as others have mentioned - much bigger than that, and you can't easily focus on everything you need to in-game. 23-24" strikes a nice balance between being able to watch the full screen and size enough to fill your main field of vision.
"How do I get rid of a Trojan Horse?" -Sailor_Kakashi "Leave it outside the gates of Troy overnight." -Davel23
It really depends on the person. I think the general thought is 24" optimize for gaming. However, after saying I never would, I upgraded to a 27" Asus MX279H AH-IPS monitor (with asus VE 23.6 on the sides) and the screen is ABSOLUTELY gorgeous and fun to game on. 5760x1080p gaming via nvidia surround is breathtaking and surreal also (yes i know the screens are slightly off due to size...fixed for the future).
Another argument people make is if you go 27" then you need a 1440p monitor which can be extremely expensive unless you take your chance with a korean one. I personally don't find 27" 1080p distracting, but again YMMV.