Well, I'm not a gamer, so I can't really advise you on this. My guess would be a smaller monitor might a better choice. Since games are graphics-intensive applications, don't larger monitors put heavier workload on the graphics card, assuming you're using a higher resolution than you would be on a smaller monitor?
But, again, I'm just rambling here. I know we have plenty of gamers in the forum. So, let's see whether someone will chime in on this discussion.