To install the Guest Additions on a linux guest system:


- install DKMS :
in a terminal (inside the virtual machine) : sudo apt-get install dkms

- mount the VBoxGuestAddition.iso file:
In the virtual machine's menu bar open the "Devices" menu
Then "CD/DVD-ROM images"
Press the "add" button in the new window
find and select the VBoxGuestAdditions.iso file(usually in C:\Program files\Sun\xVM VirtualBox)
Select it and press "select"

- install the Guest Additions :
In a terminal (inside the virtual machine)
change to the cd-rom drive (maybe /media/cdrom0)
type : "sudo sh ./VBoxLinuxAdditions-x86.run" or "sudo sh ./VBoxLinuxAdditions-amd64.run" depending on your system
restart the virtual machine



To fix the problem of screen size


After installing the Guest Additions
if necessary, quit the full screen mode (Right Ctrl + F)
look in the "Machine" menu of the virtual machine window
try "Adjust the size of the screen" or "Automaticaly adjust the size of the screen" ... (the names may be different)