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-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
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).
Shutdown your virtual box and in a terminal shell type the following command
VBoxManage setextradata global GUI/MaxGuestResolution 1366,768
the 1366,768 is just what i have set for my screen resolution on my laptop, change this to whatever your current resolution is on the machine your running. This should maximize all screens to full 100% on all guest os's
Once you have installed the VirtualBox Extension Pack (scroll until you see the Extension Pack section) use host+G or the menu option under View > Auto-resize Guest Display.
NOTE: The Extension Pack version must match the version of VirtualBox.