As far as screen size goes you just need to think about how far you're going to be sitting from the monitor. For example, I sit roughly 4 feet from my 27" and its pretty comfortable. I have also have a 40" TV mounted above my desk and I comfortably sit about 7 feet away while I'm playing BF4. Here is a link to a page detailing recommended viewing distances for TV's.
http://www.crutchfield.com/S-1ixPa2ojhYJ/learn/learningcenter/home/TV_placement.html
I personally run ASUS VE278H monitors, they are reasonably priced (on Amazon I got mine for $180, but they run around $230 on Amazon), and they have an 2ms response time, which is what you want for FPS gaming.
If you choose a 24" monitor, ASUS also makes a 24" version (VE248H) for about $165 on Amazon.
P.S. I'm not sure how much you are paying for the 270X's, but if I were you I'd get one 280X or even a 290X (ASUS has a 290X for $540 after rebate on New Egg and you get $150 worth of free games)