C:\Program Files (x86)\SquareEnix\FINAL FANTASY XIV\ffxivconfig.exe
Go to "Video Settings".
Select "Full Screen".
Then, set your "Window Size" to you monitor resolution.
When I set the game to my monitor resolution without selecting "Full Screen" I get an error.
Also, there is currently a similar post on this very page. Check out the Search Bar at the top of ZAM's page. You can find much of what you want to know by using that.