So I've installed Ubuntu inside of Virtualbox, and I resized the window of Virtualbox to be full-size, but the Ubuntu Desktop stays the same standard size. How do I make the width of Ubuntu stretch 100% across the Virtualbox window?
Inside the virtual machine, install the virtualbox-guest-dkms, virtualbox-guest-utils and virtualbox-guest-x11 packages. These "guest additions" include a video driver that will do what you ask, plus a mouse driver that interacts with the host system so you don't have to use the host key to "uncapture" the mouse; it just works when you're on the virtual machine's window.
There are 2 ways to do this:
From the VirtualBox top menu choose Devices, Install guest additions, then wait for a mock-CD ROM to appear and run the install from there, or
just type
sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11
(personal recommendation: the second (apt-get) option is very simple and quick).
