Full screen in virtualbox

When logged into the virtual machine, find the virtual Box menu (usually at the bottom center), Navigate to "Devices >> Install Guest additions". It has the shortcut key host+D

This will ask to download the .iso CD image.
Select Yes, and once the download completes it it will ask you if you want to install the same.
Select Yes again, and let it install.
Restart the Virtual machine, if needed.

Enjoy!

Installing Guest additions works perfectly fine. Just in case if someone still facing the screen size issue as i was facing, you can follow the below steps :

sudo apt-get update // just to get all system dependencies

sudo apt-get install virtualbox-guest-additions-iso // to get guest additions

and the final one
sudo apt-get install virtualbox-guest-dkms.

Please let me know if anyone still faces the issue.