Running the latest release of Ubuntu 14.10 on VirtualBox may be facing with a low screen resolution issue. After installing the VirtualBox Guest Additions form CD the problem can’t be solved. The solution is installing the latest version of Guest Additions. This is just one line command from your Terminal.
Type the following command:
sudo apt-get install virtualbox-guest-dkms
Now, you must restart the Virtual Machine and after you should get the correct screen resolution.