Can't get fullscreen resolution on Ubuntu hosted on VirtualBox

Go to the VirtualBox options while in your VM (the same place you went to to install Guest Additions) and select View -> Auto-resize Guest Display. This should automatically set your VM's resolution to the proper screen size.

In my case, I could get this fixed by changing following setting. VM needs to be shut down to change the setting.

Settings (of the VM) > Display > Graphics Controller > and select "VBoxVGA"

After struggling over and over and over .... uninstalled all virtualbox guest stuff and reinstalled dkms FIRST and then guest additions, and then was able to do View->Auto-resize Guest Display.

What finally worked:

Either substitute to root or use sudo for each apt-get:

$ sudo su -
Password: **********

apt-get remove virtualbox-guest-dkms
apt-get remove virtualbox-guest-additions

You might be told it has already been done ...

shutdown -r now

REBOOT ... NOTE: you will most likely need to powerdown from Virtualbox to reboot

apt-get update
apt-get install virtualbox-guest-additions-iso

shutdown -r now

Now in full screen mode:

Virtualbox Menu: VIEW -> AutoResize Guest Display (Host+G)

You may have to rinse-repeat until it works - took me half a dozen times! The last sequence worked, but no telling ...

Note that sometime during earlier repetitions of the process I upgraded to the most recent version of VirtualBox (which BSOD'd my PC) but I don't think that was a factor in getting the VM to finally recognize my actual screen size.

Not enough rep to comment on user1665886 & danijelc's post so I wanted to add the following:

That solution uninstalls dkms, you need to reinstall it and reconfigure it:

sudo apt-get install virtualbox-guest-dkms
sudo dpkg-reconfigure virtualbox-guest-dkms

Once I that was complete, VirtualBox automatically displayed desktop at full resolution of my monitors.

Credit to Dr. Windows this post.