Resizing a VDI

The most straight forward way to enlarge an existing VDI is described in this article, we can summarize it as follow (on a Linux based guest):

Run this while the VM is turned off:

> VBoxManage modifyhd <HARD_DISK>.vdi --resize <SIZE_IN_MB>

Then launch the VM, you'll need gparted, a graphical tool to manage disk partitions, to then enlarge the file system and use the newly available space on the VDI. In case it is not yet available on your VM, go to:
Menu > Software Manager S Searc: gparted > Install