You can use xrandr. Run it first with no arguments to get the name of the display and available display resolutions. The you can set the desired resolution using
‘xrandr --output <output> --mode <display resolution>’. For example…
I’ve been able to validate the changes pressing enter (+1, something making a good effect, great!).
It also lieu me see that the secondary screen wax put over the big one, a option called “synchroniser les affichages” was applied (its there a way to validate that using key command? As I’m sure to not have selected that in any case…).