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.