How to Install VirtualBox Linux Guest Utils
If you are trying to run a Debian based distro with Virtualbox and cannot figure out why the guest won't allow for higher desktop resolutions, or why you can't copy and paste from the host to guest / guest to host then you've come to the right place! You need to install the guest utilities from inside of the guest. After you have installed the OS, open up a terminal, and run this command as root. This will install all the guest utilities needed for 1920x1080 inside of the guest, and it also fixes copy and past functionality.
apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11
Once the packages have finished installing, reboot the guest and once it has rebooted you should be able to customize the desktop resolution for the virtualbox guest. You should also be able to copy and paste from the guest to the host, or the other way around. Installing the packages above should resolve most guest issues / annoying issues.