As I understand it, what you are wanting to do, (increase virtual hard drive size in a virtual box environment) is not possible.
The best approach is to back up and new files you have created after the creation of the virtual box OS.
Remove the current virtual box vdi and create a new one with increased space.
Though vdi cloning / backup has had mixed results it may also be something to look into if the re-creation process is going to be too tedious and time consuming
take a look at this link for more information
http://srackham.wordpress.com/cloning-and-copying-virtualbox-virtual-machines/Hope it helps
Good Luck and Safe Computing
