oracle vm virtualbox guest extension pack for full screen mode code example

Example 1: virtualbox fullscreen resolution

sudo apt-get install virtualbox-guest-dkms
sudo dpkg-reconfigure virtualbox-guest-dkms

Example 2: virtualbox fullscreen resolution

Go to the VirtualBox options while in your VM (the same place you went to to install Guest Additions) and select View -> Auto-resize Guest Display. This should automatically set your VM's resolution to the proper screen size.