After installing Ubuntu or any other guest distro based on Ubuntu like Backbox or LinuxMint as a guest OS through Oracle’s VirtualBox virtualization software, you’re often bound to experience problems with the screen resolution (the guest OS appears smaller and in the middle of the screen).
Fortunately, the solution to the screen resolution problem is simple.
Open your terminal on the guest OS and run the below command.
$ sudo apt-get install virtualbox-guest-utils virtualbox-guest-x11 virtualbox-guest-dkms
Once the installation is complete, restart your VirtualBox guest OS and the screen resolution problem will be fixed.
Important: I have tested the above method and it works fine on VirtualBox 4.3.28 with a Linux Mint 17.2 host OS and Backbox 4.3 guest OS.