List of known guests:
$ VBoxManage list vms
Start guest by name:
$ VBoxManage startvm $NAME
$ vboxmanage list -l hdds $ vboxmanage modifyhd $UUID --resize $SIZE
Resizing of VMDK image isn't supported, convert to VDI first:
$ vboxmanage clonehd $MY.vmdk $MY.vdi --format vdi $ vboxmanage modifyhd $MY.vdi --resize $SIZE $ vboxmanage clonehd $MY.vdi $MY.new.vmdk --format vmdk