Alternatively, take a look at the below posting for a similar issue.
http://planetwatt.com/newforum/index.php?topic=126.0I got away with it by making a simple xrandr script and making the window manager point to it.
To do this. Go to /etc/lightdm/
Make the resolution script
#sudo nano xrandrscript
#! /bin/sh
xrandr --output VGA-0 --mode 1680x1050 --right-of LVDS
control X to exit and save
Then point the lightdm.conf to the script
#sudo nano /etc/lightdm/lightdm.conf
edit the line that says display-setup and throw the script location there.
display-setup-script=/etc/lightdm/xrandrscript
I hope this helps...