Try using Gamescope to run it in whatever resolution you need. In Steam you can go to the Launch Options or whatever it’s called, and put in “gamescope -w 2560 -h 1600 -- %command%” which should hopefully fix it (make sure you have gamescope installed for this- if you Flatpak steam you’ll need the flatpak version, else get the one from your package manager).
Gamescope is quite the useful tool, you can read some more about it here if you want: https://wiki.archlinux.org/title/Gamescope
In my experience the issue you are experiencing is caused by scaling on Wayland versus Xwayland. Gamescope is wayland native so it should fix it. If you aren’t using Wayland, it’s still worth a try.
gamescope -w 2560 -h 1600 -- %command%x1 2025-03
Compatibility Report
System Information
I confirm:
Symptoms
My laptop resolution is 2560x1600 When I run Steam Proton with all my games, the game's quality only have 1704x1065 in the options. How can I run it as 2560x1600?
Reproduction