I just installed Ubuntu 15.04 in my Oracle Virtual VM and found that my screensize was set to 640×480 aka Too Small!
There is a quick fix though. Run the following command, shut down the machine then restart it, and it should be fixed.
sudo apt-get install virtualbox-guest-dkms
If that fails, do make sure that you are using the latest version of Oracle VM VirtualBox by upgrading.
Written by Stephen Moon
email: stephen at logicalmoon.com