I've done a post on this before, but it was for an older version of Linux CentOS it no longer works for CentOS 8 Stream.  So instead of updating the existing post, I am doing another post so it won't confuse you.  The older post is for older version of CentOS and this one is for CentOS 8

The first thing you noticed after your install of CentOS on VirtualBox is that it's pretty small even if you put it in Fullscreen mode, it doesn't not fill the whole screen at all, as a matter of fact the size stayed the same.

In this post we are going to install the Guest Addition tool in VirtualBox to make the VM fullscreen truly fullscreen. 

Here are the steps to make the VM fullscreen:

1. Log into your system as root

2. Run dnf update -y, if you have no internet access type dhclient -v to get access

3. Next we want to install Red Hat's repository of tools with this command dnf install epel-release

4. Once the update is finished run the command dnf install gcc make perl kernel-devel kernel-headers bzip2 dkms -y to install headers and build tools

9. Type in the root password and click on "Authenticate"

10. If you don't see your VM going fullscreen, then try reboot

11. Once rebooted click on "View", then "Full-screen Mode" to use your VM in fullscreen mode