I use VirtualBox for running windows on my Macbook Pro (mostly for browser testing). I recently decided to give Age of Empires Online a try but filled up my virtual hard drive that was running the system.
I the GUI interface in VirtualBox does not have any way to clone a drive and resize it at the same time, but I found this post that shows some of the underlying software does. It was dead easy to do.
The only difference for me, since I am running a mac, was the location of the command line utility that it uses. On OSX it lives at
/Application/VirtualBox.app/Contents/MacOS/VBoxManageI hope this helps someone else!