I can’t really give any more useful answer than the one I gave you several times above already, sorry. There is no single X*Y pixels answer to your question since the right answer for you depends on your personal preferences in regard to performance vs. resolution.
Rules of thumb you should keep in mind are: 1. Resolution as high as possible while still keeping the framerate high enough to be comfortable for you. 2. Square/4:3 aspect ratio if you (or vorpX automatically) can set the FOV high enough to fill the entire screen, 16:9 if you need ImageZoom to compensate for FOV. That’s fairly easy and the best advice on could give in this regard.
The trick is to set a resolution of something like 2800×1000 in the user.cfg as this will give you up to 120 fov ingame. Then set ingame res top whatever you want, it will retain the high FOV and get you better results. You will still need to create this custom res in your graphics control panel first though.
So I am experimenting with some unsupported games that managed to hook to VorpX being Everquest Titanium Edition (2006 directX 9) and WWII Online (some custom game engine) both I got to hook. Pretty neat stuff! The mighty VorpX!
I think EQ let me chose what 3d Mode , but WWII does not have the 3d mode enabled in vorpX. EQ looks like cardboard cutouts but that game allways looked like that.
So what is a good resolution to start with for such situations? 2560×1440?
I am not sure if these games would allow such high resolutions.