I have a Windows 7 with VirtualBox. I did install a fresh downloaded Ubuntu 14.04 (64bit). I did install the guest additions. I wanted to set the screen resolution to a higher value than 640x480. I did go over on/off (top right)--system settings--hardware--viewing devices.
I get only "eingebaute Anzeige" (German, Wnglish would probably be "builtin display") and only one screen resolution. I have found a howto for Ubuntu 9 and did try to edit/create an
/etc/X11/xorg.conf. But this too didn't work.