Changing resolution in command line.

To check what resolutions are available for each output device:

 xrandr -q

To change the resolution of the main screen, of the beamer, and saying that the beamer is a copy of the main screen:

 xrandr --output LVDS1 --mode 1024x768 --output VGA1 --mode 1024x768 --same-as LVDS1  

Same as before, but with beamer on the right side of the main screen:

 xrandr --output LVDS1 --mode 1024x768 --output VGA1 --mode 1024x768 --right-of LVDS1  

Changing back the main screen to the normal resolution:

 xrandr --output LVDS1 --mode 1280x800