How to change paper width in full editing mode ?

everything is in the subject :=)
I thought that was possible, but it looks like I can only change the height…
Maybe I did something wrong.

If you mean changing the paper width while in full screen, there’s a slider in the control panel that comes up if you move your mouse to the bottom of the screen. This slider changes to adjusting the height if you hold the Alt key, so perhaps you had that depressed when you were looking before?

Yes, thanks, it was locked in “change height”.
I didn’t know the function of the ALT key in this mode.