xrandr --output LVDS1 --mode 1024x600 --scale 1.25x1.25
to get 1280x750 resolution. With Unity this leaves black bands to the top and right of the screen.
The fix is easy though makes changing resolution a little faffy. Just run the above and then run
after pressing ALT-F2.
Unity rebuilds the screen to the appropriate resolution.
This post and my response on Ubuntu Forums gives more details.