So my 10GB Windows XP VM (that'll be big enough!) ... wasn't big enough.

After some googling and some help from a friend, here's how I enlarged my image.

1. Shut down the VM.

2. From a terminal, run the following:

VBoxManage modifyhd /path/to/diskimagefile.vdi --resize SIZE-IN-MB

3. Then install 'qemu-kvm' package:

$ sudo apt-get install qemu-kvm

4. Load the network-block-device kernel module:

$ sudo modprobe nbd

5. Load the VDI file as a network block device using qemu-nbd:

$ sudo qemu-nbd -c /dev/nbd0 /path/to/diskimagefile.vdi

6. Use your favourite partitioning tool (i like gparted) to open the disk:

$ sudo gparted /dev/nbd0

Adjust your active partition size (you will see the original disk size, and available space up to the new size you set (as SIZE-IN-MB above). Commit your change. Bobs your Uncle.