The screen resolution was dreadful after installing the Virtualbox tools on the Ubuntu 14.04 guest, it was awful.I crawled the web and found the following commands that should be run in the guest 14.04 virtual machine, and you then get graphic resolutions you can work with for testing.
Here they are.
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install linux-headers-$(uname -r)
sudo apt-get install dkms build-essential
Then mount the install CD image for the Virtualbox tools and install them.
cd /media/nick/VBOXADDITIONS_4.3.6_91406/
sudo ./VBoxLinuxAdditions.run
sudo apt-get install virtualbox-guest-x11
Then, once this is done you will have decent screen resolutions.
Hope this saves you some time, effort and frustration…