Steps to make Kali linux in full screen mode

Step - 1: Identifying kali linux's icons

First icon on the left is firefox
Second icon on the left is terminal

Step - 2: Open Kali's Firefox

Open firefox
Search for Kali linux source's list repository
Under Source Repository copy the command -
deb-src http://http.kali.org/kali kali-rolling main non-free contrib

Step -3: Open Kali's Terminal

Note - If for some reason, you don't see terminal icon in kali's home screen, open files (third icon on the left) and right click. Choose Open with terminal option.

In format root@kali, root is the username, kali is the hostname.
Type the following commands -

cd etc/apt
nano sources.list

Press Ctrl+O (save the content), then Ctrl+Y (close after saving)

apt update
uname -r

command uname -r will give the latest kali version running on your VM.

apt install linux-headers-*

When terminal asks -> do you want to continue? -> hit no.
Copy the first line, where you can see latest Kali version.

apt install linux-image-<latest-kali-version>
reboot
apt install linux-headers-$(uname -r)

When terminal asks -> do you want to continue? -> hit yes.

Next, click on devices > Insert guest additions CD image.

cd /media/cdrom
chmod +x VBoxLinuxAdditions.run
sh ./VBoxLinuxAdditions.run

Now, you should be able to view kali linux in full screen mode.