First thing, get your Linux OS install in Virtual Box as usual. Am going to use Ubuntu/Debian as example here.
Then, boot up the OS and open the Terminal window :
1. Login using Superuser using
sudo su
2. Update the aptitude source
apt-get update
3. Install the packages required :
a. apt-get install build-essential
b. apt-get install linux-headers-generic
Once completed, click on the VBox “Devices” menu and “Install Guest Additions”.
Then, go back to the Terminal window :
4. Make a new mount folder,
mkdir /mnt/vbox
5. Mount the Guest Additions to the newly created folder,
mount /dev/scd0 /mnt/vbox
6. Install the Guest Additions, depends on either you’re using x86 or 64bit workstation,
cd /mnt/vbox
./VBoxLinuxAdditions-x86.run
./VboxLinuxAdditions-amd64.run
7. Reboot and you shall able use full screen as you like.