I have been playing with Ubuntu lately and for some reason I can't resize the resolution on my guest machine.I have several other machines and they work find, but when I look on this particular guest machine it did not work. this whole mess started for me after I upgraded my virtualbox 5 to version 6. Even my clipboard did not work.

I searched everywhere to find the answer. Most of the posts i found said to installed guest additions. I did all that. So if you are on the same situation as i ma, lets try this and see if it works. so this is what I did.

update your packages

$ sudo apt update

$ sudo apt-get upgrade
This will take a while!

Reboot your guest machine

install guest additions

Power off the machine [Don't reboot]

now that that machine is powered off, go to settings > display >
in the Screen tab select: Graphics Controller VBoxVGA

Start your guest machine and you will see it works!

I looked everywhere and this is the only thing that work for me. I wrote all my notes and this is what I have. Hope this helps you fix