Hi Draken!
Go to your nvidia panel, look for resolution settings. One of the resolutions in the drop down menue should be marked as native (that's what my Nvidia panel shows). This is the resolution your monitor can afford, no matter what your graphics card can (...and mine can do more, but that is not shown in the dropdown menue).
All you'll ever see is the maximum resolution of the screen. Simply as there are
physical not more dots available. And that is all you can "print" by a screenshot. That can't be doubled by your graphics card. One physical dot is one physical dot.
You might change that screenshot with Gimp or another tool of your choice, so you can post/print it in another resolution than it was captured. But that is then a file.
A screen with a maximum resolution of 1920 x 1080 will never show it in 2560 x 1440, because it simply can't due to its physical limitations....
I once played around with wallpapers on my screen, which is a native 1920 x 1080 one, but whenever I tried a wallpaper with a higher resolution I earned a black screen. Game over.
You can try to change the resolution inside the conf.ini. I've just tried that - the launch get stucked, even though the wallpaper background shown up a bit bigger in the middle of the screen. My 21" monitor is too small for that
.
Have a nice day,
BigMug