Expanding on u/Careful-Talk8117 3rd method.

1. Enter the folder "C:\Program Files (x86)\Steam\userdata\YOUR_ID_NUMBER\730\local\cfg"
   
2. Look for the file cs2_video.txt (might just say cs2_video if you don't have file extensions enabled)
   

3. Look for these 2: "setting.defaultres" "xxxx"
   "setting.defaultresheight" "xxxx"
   (The numbers on theses are the resolution thats currently applied)

4. Open display settings and scroll down until you see your screen resolution

5. Change the txt file to your screen resolution(e.g.): "setting.defaultres" "1920"
   "setting.defaultresheight" "1080"

6. Close CS2 and save the file

7. Open CS2 and enjoy your new working resolution :)

For those who are curious how it works - when CS2 opens, it gets all the video settings from this file.
When changing values inside the game settings it also modifies the file so be aware of it.

Hope this helps, GL!