That resolution is of the same aspect ratio, so it shouldn't look too bad.
Originally Posted by CatKiller
First you'll need to generate a modeline for that resolution:
which should output something like
then create a new resolution mode that uses that modeline (the part in bold, yours might be different so make sure you run the gtf command first):
# 960x600 @ 60.00 Hz (GTF) hsync: 37.32 kHz; pclk: 45.98 MHz
Modeline "960x600_60.00" 45.98 960 1000 1096 1232 600 601 604 622 -HSync +Vsync
now attach the new mode to the appropriate interface:
xrandr --newmode "960x600_60.00" 45.98 960 1000 1096 1232 600 601 604 622 -HSync +Vsync
then switch to it using
xrandr --addmode LVDS 960x600_60.00
xrandr --output LVDS --mode 960x600_60.00
You can script all your resolution changes depending on the active interface, but that's a little beyond the scope of the question. Here is a page with some info on persistence in xrandr changes: https://wiki.ubuntu.com/X/Config/Resolution.