Boot the VM with a utility like PartedMagic, use GParted to decrease the size of the Virtual HD, then use G4U that is included in that distro to make an image of the VM's HD to a network drive or add a 2nd Virtual drive to your VM and image to that, then shut-off the VM, delete the original HD, Create a new, smaller one (make sure it isn't smaller then the size you reduced it to with GParted before), and then again boot using the PartedMagic iso, and restore the image you made to the new HD.
http://partedmagic.com/
