ubun2

The VDI files reside on the host system.
They are typicall seen by the guest
systems as hard disks a specific way.  a "geometry" if you will.
When creating images,
its size has to be specified which determines this fixed geometry.
It
is therefore not possible to change the size of the virtual hard disk
later.
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Possible workaround:
You COULD, try this possible workaround (just throught of it though)
1.  Create the new, LARGER Virtual Disk.
2.  Add the SMALLER virtual disk as a secondary IDE drive.
3.  Boot Ubuntu from a LIVE edition on the new VM
4.  Copy the contents of the SMALLER Disk to the LARGER disk
5.  Attempt a reboot of the VM without the live cd in.  if it works, it works. if it doesn't... can't say i didn't try.
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Another Good Solution:
REMASTERSYS is an all around great tool that you can use, to reinstall your ubuntu system from scratch.  What's great is that you can choose to backup Ubuntu, plus all the packages, libraries, and applications including personal
data to a live cd or dvd that you can use anywhere and install.  You can also use it to make a distributable OS that you can copy and share with your friends.
Here is the Remastersys page:
http://www.geekconnection.org/remastersys/remastersystool.html
Here is some help to install too:
http://www.ubuntugeek.com/creating-custom-ubuntu-live-cd-with-remastersy...
 
What you could feasibly do:
1. install remaster sys to the guest OS
2. backup the entire VM to an ISO file
3.  Make a new Virtual Disk Image that's much larger
4. Boot a new VM running the exported ISO file of your old VM install
5.  Install it to the new VM