http://www.webupd8.org/2011/02/how-to-easily-resize-virtualbox-40-hard.html

VBoxManage showmediuminfo YOUR_HARD_DISK.vdi
VBoxManage modifymedium YOUR_HARD_DISK.vdi --resize <newsize-mb>
    300000

+gparted live

http://derekmolloy.ie/resize-a-virtualbox-disk/