1. Set Windowed mode on
2. Get Autohotkey
3. Make a script like this:
^+a::
WinSet, Style, -0xC00000,a ; remove the titlebar and border(s)
WinMove, a, , 0, 0, 1600, 900 ; move the window to 0,0 and resize it to 1600x900 (or whatever you use)
return

4. Run the autohotkey script
5. ctrl+alt+A on an open FFXIV client
6. ????
7. Profit