If you can run at 120 FPS then your res or textures arent high enough. 60 fps/hz is fine and I will hold that logic until we have perfect life-like graphics and then have enough processing power to increase the framerate.
Theres still issues with scaling on very high dot pitch and 21:9. Thats why i recommend 32" 1440p as the best choice.
If you must go 21:9 then you want it to be 34". smaller sizes have ridiculously low vertical height. You want to go back to a 17" monitor when stuff isnt displayed in 21:9?
Samsung S32D850