The answer to setting a custom resolution in Arkham Origins is to locate the "GFXSettings.BatmanArkhamOrigins.xml" in your user account, under: C:Users\"your account name"\Documents\WB Games\Batman Arkham Origins. Right click and choose EDIT. You should be able to find your current resolutions numbers in there, and are able to change them to whatever u would like. All of the changeable video options are in there.

This is handy for those that want to run the program in windowed mode, like me. This makes it much nicer to be able to flip back and forth between different things, while still running the game. Mine is set for 1915x1020, and the viewing area fits nicely on my 1080 monitor.

IMPORTANT!Make a back-up of the file before you make any changes. Just in case you screw something up, you can just paste back the original and try again. I warned you.

Also, sorry to the original poster, it's not exactly an answer for you. Playing around with these setting might get you the result you are looking for though. But I found this by searching for that, and bet this may be helpful to many who also find it by the same means.