What resolution do you play at now?
I'd say for 1080p then you want a 24" display, 1440p then 24-27", 4K then 27" and above. 4K and high refresh rate is hard to push unless you have a powerful PC. What are your specs?
What I'd recommend is a 1440p 144hz monitor. As everyone likes to say, it's the "sweetspot". They are cheaper than 4K obviously. Resolution is clear enough but not as demanding as 4K. And that lets you chase higher frames. 144hz is the standard on high refresh rate gaming monitors. You can get 120hz TVs or 165-360Hz monitors. I honestly wouldn't bother for anything beyond a 175Hz monitor unless you want to competitively play esports stuff. Even though monitors will advertise as 165-175Hz you should probably just run them at 144hz. My monitor is advertised as 165hz but I run it at 144hz.
If you're getting a 144hz display then Gsync is a must. In fact, you'd probably struggle to find a non VRR high refresh monitor. If you have an AMD GPU you are generally looking for anything that has Freesync. With Nvidia you should make sure the monitor you want to buy is on this list:
Search for GeForce G-SYNC gaming monitors.
www.nvidia.com
Gsync Compatible monitors are basically Freesync monitors that have been tested to have basic VRR functionality and quality. You can use an Nvidia card with them.
I think that might answer most things if you're new to monitors. If you have any questions I'll try to help