You're just missing a step in the AutoHotKey script. Here's mine:

^+a::
WinSet, Style, -0xC00000,a ; remove the titlebar and border(s)
WinSet, Style, -0x40000,a ; remove sizing border
WinMove, a, , 1680, 0, 1360, 768 ; move the window to 1680, 0 and resize it to 1360x768
return
Edit "1680, 0" to be the desktop coordinates you want the upper left corner of the screen to be (0, 0 for your primary display) and "1360, 768" to be the resolution you're running the game. Edit "^+a" to be whatever keyboard shortcut you want for it.