Make Ubuntu full screen in virtual box

My friend told me that he installed Ubuntu 14.04 32 bit on his Virtual box but the screen is too small
and he can't make it as full screen so I tried to solve this issue and here is the steps
1- Download  VBoxGuestAdditions ISO for the virtual box version which you have
   EX: if you have latest virtual box version 4.3.20 you have to download     :VBoxGuestAdditions_4.3.20.iso
2- Open your ubuntu virtual which already installed and run this command while you are connected to the internet

sudo apt-get install dkms

3-  Insert the iso VBoxGuestAdditions into the virtual machine using virtual box

4- Open terminal and go to the path of CD
  "cd  /media/{user name}/VBADDITIONS_{version number}"

5-Run this command

sudo sh ./VBoxLinuxAdditions.run

then restart the virtual machine and it will works fine full screen mode (HOST+F)
Source of commands
https://www.virtualbox.org/manual/ch04.html#additions-windows