Simplest guide to expanding your virtual hard disk in virtual box: http://www.addlime.com/server-admin/resizing-virtualbox-virtual-machine-debian-hard-disk-lvm/
Some other links:
- http://jutanclan.blogspot.in/2013/06/extending-size-of-virtual-disk-in.html
- http://www.oracle-base.com/articles/vm/virtualbox-extend-disk-and-file-system.php
- http://www.munzandmore.com/2013/ora/extend-a-centos-oracle-virtualbox-image-with-more-disk-space
- http://derekmolloy.ie/resize-a-virtualbox-disk/