Having just recently set up Ubuntu again in a Virtual Box VM on my Windows 7 PC, I ran into an issue where full screen mode wasn't actually full screen.
To enable full screen mode, simply have your VM running Ubuntu and click Devices -> Install Guest Additions.
This will mount a disk on your Ubuntu machine. Simply open the Terminal and run the autorun.sh shell script.
Reboot and your set. Host + F will now expand your Ubuntu VM to full screen!
