Ok, here is another issue about xrandr.
I tried xrandr -q to find out the correct size and got
Xlib: extension "RANDR" missing on display ":0.0"
And then I tried xrandr with some flags, e.g. -o -s etc, and got
Can't open display 0
Is there something we have to set in /etc/X11/xorg.conf? Or something else?
Thanks!