If you are looking to play "fullscreen" without the fullscreen alt-tab hassle, try the following:
1) Open your aRO Folder
2) Open "dinput.ini" file in Notepad
Set your MouseFreedom variable = 1
Set your WindowLock variable = 1
If you are looking to set custom resolutions you may do so like this following setup (";" is the comment line prefix, so if you prefix the line the client will ignore the instructions. Useful for example, maintaining a 1280x720 setting for when I need to Fraps in 720p native resolution, just remove the semicolons)
; Override RO window position and size.
; Ignored if WindowWidth = 0 or WindowHeight = 0.
; Command line switch: -wX,Y,Width,Height
;WindowPosX = 359
;WindowPosY = 159
;WindowWidth = 1280
;WindowHeight = 720
Note: My native resolution is 1920x1080, the X and Y positions I used are to center the client window given a 1280x720 window on my screen.
X and Y positions do not follow Cartesian Coordinates, but rather the top left corner of the screen (0,0).
Hope this helps and is not too confusing.