I reckon 24' to 32' TVs'..32' being the max for playing COD or FPS games cos' you don't want your eyes to dart left and right and up and down just to digest all the action onscreen,which will limit your full potential or skills.
And preferably a monitor made especially for FPS like with a fast 2-5 ms response time,1080P Hi Def for crystal clear vision.These monitors are quite affordable as well so that's a bonus.
I rape shit up on the 55. I sit about 12 feet away in a full reclining theater chair. It's just a perfect viewing angle, hard to explain. You don't have to resort to a tiny screen to be good. I still rely on this TV for tv shows and movies. I can play all day without straining my eyes or getting a headache.