Expanding drive space in VirtualBox has gotten easier! Just clone the VDI (this step isn’t required, but if something goes wrong you’ll be happy you did it), then zap the clone with this:
VBoxManage modifyhd <absolute path to file> --resize <size in MB>
Fire up the clone. If you’re running a modern Windows OS you can use Disk Manager to expand the partition to fill the volume. Otherwise check out gparted.