Your Native Resolution: 1600×1200
Set your in-game display resolution to 1600×1200.
To render at the following 3D resolutions, either set your RenderQuality value in UserOptions.ini OR Render Quality slider in the in-game options to the following values:
|3D Resolution||RenderQuality=||Render Quality Slider|
It is advisable to set the option manually by editing UserOptions.ini as it will give you the exact value.