First you need to check the list of supported modes. When running the command
it shows me (for my netbook with external monitor) two displays: LVDS1 and VGA1. VGA1 is the external monitor but xrandr doesn't recognize it's native resolution.
We can generate the mode we want to add with the following command:
cvt 1680 1050
The output should be Modeline and then some text. We create the new mode with the following command:
xrandr --newmode "1680x1050_60.00" 146.25 ...... + vsync
The last step is to associate it with the display:
xrandr --addmode VGA1 "1680x1050_60.00"
Now you should be able to select the correct resolution for the external monitor