Custom Full Screen Resolution??????

I am aware there is likely already a thread for this, but I am looking for ways to make the game visually more enjoyable while maintaining playability.

Currently I am running 1280-720 with high textures averaging 15-75fps.

Here is what I'm trying to do:

Significantly lower my resolution while staying in FULL SCREEN 16:9 aspect ratio.

I've tried inputting every 16:9 res I could find into the config, and it always automatically switches me to 600x800 4:3 res. However, it works fine with 480-640 4:3 res. But as mentioned before, my screen is 16:9 so it obviously looks like shit.

Is there a list of compatible full screen resolutions by chance?

Any help is very highly appreciated!