vorpX can set the best possible resolution automatically for many games. To take full advantage of this new feature add all resolutions listed in the “Custom Resolutions” section of the vorpX help (or the according sticky thread here in the “General” forum) to your graphics driver and after doing so select a preferred graphics quality on the “Direct VR” page of the vorpX ingame menu.
For games that do not support this new feature 1600×1200 or 1920×1440 are good resolutions when using vorpX. In such a case you can set the resolution either before you run the game with vorpX or while you do so.
If you want to set the desktop to a 4:3 res, you can do that in the Windows options.