you try going to yer start menu and going to square enix folder and going to the ffxiv beta version config? it auto runs a configuration and sets yer resolution and gives you an option to turn on full screen...worked great for me.
I have the same issue... Full screen used to work. Now when I set it to full screen in the config it loads up a black window but never actually maximizes the game. It just stays like that until I end the task and only allows windows mode to work. Any help would be greatly appreciated!