Is it possible to run the game windowed at 1920x1080?

Solution 1:

Edit the system.ini file with a hexadecimal editor like HxD. I'm not sure will this help or not, because I don't have DNF, just telling how to edit the file.

Additionally you can try launching the game with screen resolution set in command line arguments. Right-click DNF in Steam, press Properties, click Set launch options and put the following line: ResX=1920 ResY=1080

Solution 2:

I tried passing ResX=1920 ResY=1080 and -width 1920 -height 1080, but it didn't work. I tried resizing the window using the mouse, but that is not possible either.

What finally worked was using an external tool to resize the window after the game is launched. I used Borderless Gaming, and then Duke Nukem Forever got the resolution I wanted, while still in windowed mode.