Greebo's post is a great summation of monitors for gaming IMO. As well as TFT, 144hzmonitors.com is another gaming monitor site that gives accurate advice.
IMO 144hz/165hz is the most important feature in a gaming monitor. Size is personal pref, although I've found between 24, 27, and 34" I think 27" to be optimal, regardless of desk size/etc. TN/IPS, regarding that, I find that TN panels are great for competitive FPS games like CS GO or Doom/Quake/etc, while IPS is probably the best option for a game like AH or DCS where the instant single shot weapons aren't as critical. IPS will in almost every case look much better in all games - I have the ROG Swift Asus 27" in both TN and IPS, one is 144hz, the 165hz, more or less the same in terms of refresh, but standing side by side, the IPS variant looks better in every case, in every game. I wouldn't even have bought the TN except it was out for a year before the IPS Swifts came out. I have the 34" Swift as well, it's 100hz/IPS/Gsync, and is excellent, but some might find it a bit too wide/large - if you're into 3 screen, you won't notice that IMO.
Gsync = I've not tried Freesync so I can't comment on how it works, Gsync does improve things as Greebo described, but honestly, the last 144hz monitors I had didn't have Gsync, and I didn't really miss it that much. If $ is a concern and you can't get a panel that has it all - 144hz/IPS/Gsync, for me Gsync would be the first thing I sacrificed for cost, and again, I have 4 gaming monitors right now, the 3 ROG Swifts and an Acer 28" 4k Gsync 60hz.
Asus has a new monitor coming out in q2 this year, another in the ROG/Swift series, PG27UQ, 27", 4k, Gsync, and 144hz@native 4k res. Acer and Benq have competitors with the same coming out as well. If $ is no object, these will be the best panels you can get, which have it "all", 4k capability@144hz, IPS, and Gsync. It's been 1440p max for 144/165hz up until these new models.
Rebel - I'm not 100% on the mechanics or details, but this is my experience - I went from 24" Asus 144hz monitors to 27 for my main AH setup, and I sit at the same distance, as I'm a "forearm rester" when using my sticks, ie my right forearm is on the desk, not hanging in space when I use the stick. IE I know I'm at the exact same distance with both LCDs - I didn't notice a big change in how I saw targets or fought/etc, but obviously can tell it's a larger picture. If I flip my 27"1440p monitors to native 1440p, I do find spotting dots a bit harder, and find the target planes smaller, so I fly at 1080p if I'm not using VR.