After I installed Unbuntu on Oracle VirtualBox I could not change the resolution of the guest OS.
The solution is to open a terminal window.
run these commands. It will ask you for your password because of the sudo command.
sudo apt-get update
sudo apt-get install virtualbox-guest-utils virtualbox-guest-x11 virtualbox-guest-dkms
shout out to this dude on youtube.com that pointed me to the solution.