Extend Virtualbox Ubuntu Window Size

# Extend Virtualbox Ubuntu Window Size
# Press Ctrl + Alt + T to open terminal
# execute
sudo apt-get install virtualbox-guest-dkms
# Input your password when asking
# Type Y then Enter when asking
# Restart VM
# Done

Update : 

The method above doesn't work for me in newer version of virtualbox, another way:

Run Command

# Update repo
sudo apt-get update
# install linux headers
sudo apt-get install linux-headers-$(uname -r)
# install build-essential and dkms
sudo apt-get install build-essential
sudo apt-get install dkms

Then click 'Device-> Insert Guest Addition CD Image' at menu bar and follow instruction.

Reference:

How do I install Guest Additions in a VirtualBox VM?

cannot change screen size from 640x480 after 14.04 installation on VirtualBox OSX [duplicate]