## From the xrandr man page



RandR version 1.1 options
       These  options  are  available  for  X servers supporting RandR version 1.1 or older. They are still valid for
       newer X servers, but they don't interact sensibly with version 1.2 options on the same command line.

       -s, --size size-index or --size widthxheight
              This sets the screen size, either matching by size or using the index into the list of available sizes.