For months I've been living with a 1024x768 screen size on a 1366x768-pixel screen after a system upgrade, and not wanting to wade through all the obsolete advice found on Google. Finally I tried again last night and got it working with this:
jcomeau@intrepid:~$ cat /usr/share/X11/xorg.conf.d/20-ati.conf
Section "Device"
Identifier "ati"
Driver "radeon"
VideoRam 16384
EndSection
last updated 2012-12-05 00:48:29. served from tektonic.jcomeau.com