First off, make sure the X server supports it. Many of the new ultrawide screens use multiple video connections and current xorg treats them as different monitors, often with unfortunate results because it's not as straightforward as putting half of the screen on one and half on the other. Last I heard, they were still arguing over how to change xrandr and the low level xinerama stuff to handle it properly, and what level of application level (this includes xmonad) changes might be required to support them.