Steps required to enable full-screen graphics on Ubuntu host are: sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11 sudo apt-get install dkms sudo apt-get install build-essential module-assistant sudo m-a prepare In the Guest system, go to the top menu, and click Devices->Insert Guest Additions CD Image. This should mount the CD image. Run the installation.
Reference: http://askubuntu.com/questions/451805/screen-resolution-problem-with-ubuntu-14-04-and-virtualbox