I personally find 23" - 24" perfect for gaming at 1080p.
With that resolution, it would be too pix-elated at 27" - 29", and you would need more GPU power to push 1440p, and for me personally 27" and above are to big for competitive FPS gaming.
Well, this is down to personal preference and is also tied to the rest of your PC.
If your system can handle higher resolutions (2K, 4K) then you can afford to get a higher resolution screen which is usually (but not necessarily) physically bigger.
I personally find the following most comfortable to play on.
1080p = 23" - 24 "
1440p = 27" - 29"
2160p = 30" - 32"
I personally find 23" - 24" perfect for gaming at 1080p.
With that resolution, it would be too pix-elated at 27" - 29", and you would need more GPU power to push 1440p, and for me personally 27" and above are to big for competitive FPS gaming.