This might work on other systems too, I tried it on Ubuntu 14.04.1.

In VirtualBox the screen for Ubuntu was very small and it was difficult to see what’s on the screen. To fix this issue, you need to install the following packages on the guest (VM) box:

sudo apt-get install virtualbox-guest-dkms virtualbox-guest-x11

If you get this error:

The following packages have unmet dependencies. virtualbox-guest-x11 : Depends: xorg-video-abi-15

You will need to remove this package (note: I am not sure what all this removes, it was a lot – use caution):

sudo apt-get remove libcheese-gtk23

Then install these

sudo apt-get install xserver-xorg-core-lts-trusty

And

sudo apt-get install xserver-xorg-core

Then try to install the virtualbox packages, as the first step:

sudo apt-get install virtualbox-guest-dkms virtualbox-guest-x11

Once they install properly, restart the VM and the screen should be normal size. You might need to increase the Display memory in VB before starting the VM.