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
Where YOUR_HARD_DISK.vdi is the VirtualBox hard disk you want to resize and SIZE_IN_MB is the new virtual hard disk size, in megabytes. For example, the following command will resize the VirtualBox hard disk called "natty.vdi" to 12000 megabytes:
VBoxManage modifyhd natty.vdi --resize 12000
That's it! The process takes just a few seconds and you should now have a resized VirtualBox hard disk.