Personally, I would wait for the UHD market to come even further down in price before I bought one. So any TV that is 1080p and probably 120hz is the sweet spot. In these models, the best for gaming is definitely not plasma, many would lead you to believe that, but it is absolutely not true. Plasma can show images at up to 600hz, but what is the point of that? What signal is transmitted at that speed? All it winds up doing at that point is screwing with the picture and blurring things, definitely not worth the super quick refresh rate. Not only that, but with the current speeds of signals in general the absolute tops you would probably want would be a 240hz TV.
OK, now moving away from that and looking at brands, assuming that the TV you want is LED with smart capabilities, Samsung is a great TV. They are on the higher end of the spectrum price-wise, but also hard to beat quality-wise. Sony has gone downhill (over-priced for pretty lateral quality to other brands), I would honestly stay away from them. If you are looking for high-end quality at a pretty reasonable price I would say go LG. If you are looking for pretty darn close to high-end quality for a great price, I would go with Vizio. I personally own a TLC that I got on a black Friday steal last year and it's OK quality, but can't stand up visually next to my Vizio. Most TVs now have a game mode where they will kind of lose a slight amount of graphical quality to allow for faster visual transmission, so I wouldn't worry too much about that.
As far as size goes, as stated in a previous post it really depends on the distance you are sitting from the TV. If you are like me and sit only a few feet from your gaming TV I would not go bigger than 32". As you sit further and further away, naturally your screen size should grow along with the number of feet you are playing from. I hope this post helps even a tiny bit!
Edit: I just want to state why to stay clear of 4K for now. So my reasoning is that very little is transmitted in 4K currently, and it will take another few years for the market to shift entirely to 4K. So you may be ahead of the market for awhile, but you wouldn't be getting all that much value out of the purchase. As far as the hz of the TV, the reason 120 is the sweet spot in my mind is because that is double what you may be getting at the top end of videogame system visuals currently which is 60fps. This would allow your tv to refresh at a great speed without interrupting visuals to the point that it distorts them in any way due to the difference in refresh rates. Also, just for reference cable TV for example will top out at about 60fps, most signals are 24fps, and some are broadcast at 30fps. So honestly at this point even a 60hz tv would do, but I would definitely recommend going higher than that with a 120hz or 240hz, it will give you the same visual fidelity while refreshing at a faster rate - making it more future-proof for the next few years.