I've just installed Ubuntu 14.04.3 in VirtualBox 5.0.14, and got the usual problem of only being able to get 640x480 screen resolution.
I ran
sudo apt-get install virtualbox-guest-dkms
, restarted virtual box; no change.
I ran Xdiagnose with all diagnostic options checked - now I have a choice of 640x480 and 1024x768. Well, that's better, but still not good enough.
I go to System Settings | Software & Updates | Additional drivers, and under InnoTek Systemberatung GmbH: VirtualBox Graphics Adapter select "Using x86 virtualisation solution-X11 guest utilities from virtual-guest-x11 (proprietary)" and click apply. The computer thinks about it for a few moments, then changes my selection back to "Do not use the device".
So what next? How do I get bigger screen sizes?