VirtualBox 4.0 got a very cool new feature that allows you to easily resize a hard disk in just a few seconds. Previously, you had to install Gparted to do this and the procedure was quite slow.
In VirtualBox 4.0+ (see how to install VirtualBox 4.0.x in Ubuntu), to resize a VirtualBox hard disk image (.VDI) firstly locate the folder where the .vdi you want to resize is located – this should be under ~/VirtualBox VMs or ~/.VirtualBox/HardDisks. Then open a terminal, navigate to that folder (« cd /FOLDER/PATH ») and run the following command to resize the .VDI:
VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB
VBoxManage modifyhd natty.vdi --resize 12000