Good afternoon and thank you for posting on the Final Fantasy XIV Technical Support Forums.

The Fullscreen option should allow you to choose any resolution usable on your desktop. You mentioned using a custom resolution, how did you go about creating a custom resolution? I ask because I tested this and was able to make one using the Nvidia Control Panel and set the game to 1920x1280 in Fullscreen.

It sounds like those other resolution options aren't native to your screen.

We hope we can be of assistance in helping you enjoy the exciting world of Final Fantasy XIV!