Resizing a VirtualBox disk
Resize a VirtualBox guest Linux VDI Disk under Windows Host
It is April, 2013, I am using VirtualBox 4.2.6. The host OS (the one that is running VirtualBox) is Windows 7 and the guest OS that I wish to re-size is Ubuntu. Please backup everything before continuing as something could always go wrong.
Anyway, here are the current steps to re-size a VirtualBox disk, where Linux is the guest OS and Windows is the host OS.
I mostly followed this procedure, but installed gparted on a Debian VM.