Take in look in your Documents folder then "My Games\FINAL FANTASY XIV - A Realm Reborn". There's a ffxiv.cfg file which you can open/drag into notepad.

Recommend make a backup of the file first, but a few lines down is ScreenWidth and ScreenHeight. I don't know if it'll work, but you could try changing those numbers.

However, it's likely that making any updates to the graphics settings in the game with overwrite your changes.

Apologies if it doesn't work, but worth a try?