I recently install Ubuntu 14.04 into a Oracle’s VirtualBox. This went fine, but I was unable to get the desktop to resize to my host machines.

I selected “Devices >> install guesst additions”. It installed and I restarted, but still nothing. I tried manual ways and bunch of stuff, but nothing worked till I ran this command in terminal.

sudo apt-get install virtualbox-guest-dkms

Restarted the virtual machine and everything worked.

This link helped me.

http://ubuntuforums.org/showthread.php?t=2198058

I hope this helps someone else.