The maximum resolution the rPi can output (through its VideoCore IV chip) is 1920x1080. This is a 16:9 aspect ratio. This link on raspberrypi.org shows that it is possible to get a 21:9 aspect ratio working - There are people that report being able to run at 2560x1080 on OpenELEC.
You will need to play with the HDMI timings in
tvservice -m DMT
will list the necessary parameters for your monitor.
The format of the new lines you need to add to config.txt is:
hdmi_cvt=<width> <height> <framerate> <aspect> <margins> <interlace> <rb>
width width in pixels
height height in pixels
framerate framerate in Hz
aspect aspect ratio 1=4:3, 2=14:9, 3=16:9, 4=5:4, 5=16:10, 6=15:9
margins 0=margins disabled, 1=margins enabled
interlace 0=progressive, 1=interlaced
rb 0=normal, 1=reduced blanking