A freshly installed Ubuntu as guest under VirtualBox allows only 640*480 resolution - nothing you can really work with today.
The normal hint to get smoother graphics, including higher resolution, is to install guest additions for VirtualBox.
As normal install of guest additions also allows for shared folders to be created,
which is something I want to avoid for security reasons at least in mostly web targeted VMs, I did not install them.
I have found a way to enable 800*600 and 1024*768 via enabling debugging options in xDiagnostics, which is half-way to ok.

I have NOT RTFM on guest addition installation options to see if it is possible to select which part of the additions will be installed.
At least one post I found seems to hint that way. Avideo pointing to a combination of "sudo cvt" and "sudo xRandr" did not work out.
Another hint:
"Go To the 'File' menu, and activate 'Environment setting (P)'. Select 'Display', and change the setting for 'Maximum guest screen size' to 'hint', which allows you to set an arbitrary size for both width and height (eg., 1920 and 1200)."
did not work for me, as under 14.04.4 I did not find a environment setting under the "File" menu called up from second icon and did not find another "File" menu call up...

Facepalm coincidences stopped by more coffee and this post here...

Has anybody here
found a way to enable other screen resolutions under Virtualbox without installing guest additions or
is certain that there is a way to disable the installation of shared folders when installing guest additions ?

tia

thomas