Stomach was acting up, which is one of the reasons I'm still up. Sucks trying to sleep when you need a trip to the loo every 10 minutes. =P
Anyway I use a 24" monitor, and I wouldn't want to have it any smaller with that. It has 1920x1200 resolution and supposedly 4 ms response time. Basically so long as it's not much higher than 6, it should be plenty for gaming. Although don't get too slow of a response time, since you can notice a... ghosting effect? Can't remember the exact term, but during action packed scenes in movies, or fast paced games, you'll notice the monitor can't quite keep up so to speak.
Apart from that I'd say anything between 21"-27" where for me 24" is the best. Personal preference after having used that for quite some time. Although if you ever plan on having 2 monitors, it might not be wise to go with 27", unless you have a lot of space on your dekstop.