Ok I misunerstood your point, you ment the desktop menu. This permit to lower the icon size which has an effect to kstars but make all desktop very very small.
I finally managed to get it right: in my /boot/config.txt I changed
<<
hdmi_group=2 % DMT mode
hdmi_mode=82 % mode 1920x1080
hdmi_ignore_edid=0xa5000080 % ignore edid informations (if not set the screen goes to 1024x768)
>>

to

<<
hdmi_group=1 % CEA mode
hdmi_mode=16 % mode 1920x1080
#hdmi_ignore_edid=0xa5000080 % ignore edid informations (if not set the screen goes to 1024x768)
>>

and all works fine without lowering the desktop icons.

This is all misterious to me but the problem is solved!

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