So I decided to enlarge it.
Create a new vdi in ~/.VirtualBox/VDI by running:
$ cd ~/.VirtualBox/VDI $ VBoxManage createhd -filename new.vdi --size 15000 --remember
where size is in MB, in this example 15000MB ~= 15GB, and new.vdi is name of new hard drive to be created.
Next the old vdi needs to be cloned to the new vdi, this may take some time so wait while it occurs:
$ VBoxManage clonehd old.vdi new.vdi --existing
Detach old harddrive and attach new hard drive, replace VMName with whatever you called your VM:
$ VBoxManage modifyvm VMName --hda none $ VBoxManage modifyvm VMName --hda new.vdi
Boot the VM, run Partition Wizard to resize the partition on the fly, and reboot.
Remove old vdi from VirtualBox and delete it:
$ VBoxManage closemedium disk old.vdi $ rm old.vdi