Increase VirtualBox Hard Disk (VDI) Size

1. Shutdown the VM for which you want to increase the hard disk size.
2. Make sure PATH environment variable has VirtualBox location included:
C:\Program Files\Oracle\VirtualBox
3. In the command prompt, cd to location of .vdi file for the virtual machine and execute below command (size is in MB):
VBoxManage modifyhd ocm1.vdi --resize 30720
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
This takes care of increasing size of vdi to 30GB.

http://www.turnkeylinux.org/blog/extending-lvm