I am putting here some notes for posterity
In the current version of armbian (testing H6) I use X11 / Xorg only reaches 1024x768, but my display reaches 1440x900.
To add this new resolution to the list of Settings/Display you have to give these commands:
# xrandr --listmonitors
(this command serves to see what it's called, the hdmi output)
# cvt 1440 900
# 1440x900 59.89 Hz (CVT 1.30MA) hsync: 55.93 kHz; pclk: 106.50 MHz