First you need to determine the size of your screen (e.g. its resolution)
Then you go into Start menu -> Square Enix -> FFXIV
and open the config program
There is a button that syas Graphics, click on it and change the resolution to the right one.
FFXIV: Karamethien Seraphus
Still a Seraph Samurai Wanna Be
Fishing AF - Complete, SAM AF - Complete, RNG Af - Complete, RDM AF - 1/6