In order to play Killing Floor with a custom resolution, you'll need to open the KillingFloor.ini file located in the game's files, more specifically, the system file. For most people, it's in C:\Program Files (x86)\Steam\steamapps\common\KillingFloor\System (you can copy this and paste it in file explorer).

Once you're in the file, search or find WindowedViewportX underneath [WinDrv.WindowsClient].

There, you will see these settings:

WindowedViewportX=
WindowedViewportY=
FullscreenViewportX=
FullscreenViewportY=

These settings alter the resolution in your game, and if it wasn't clear at first, the WindowedViewport settings change the windowed resolution, while the FullscreenViewport settings change the fullscreen resolution. X represents the horizontal pixel count, with Y representing the vertical. With this information, you can change the values after the = to make your custom resolution.

From my experience, the game doesn't give you the option to play higher 16:9 resolutions than 1920x1080, meaning even if you wanted to play at 2560x1440 or 3840x2160, you'll need to use this method in order to do it. However, using this method allows you to play at many more resolutions other than the regular ones you're shown in the settings (which is the reason I made this guide). I like using this method to play the game with a higher aspect ratio than 16:9.

That's it! Hopefully you leave this guide with what you came here for. If you have any questions, comments, or concerns, feel free to leave them below!