Ubuntu – Portable virtual machine with preinstalled Ubuntu for Windows

On Debian based distro you should use virt-resize instead. This handle pretty much everything under the hood now. Let's assume your image is called Win7 (why not?). First thing make sure your VM is shut down:

Install the tool:

# apt-get install libguestfs-tools

Get the location of your VM disk:

# virsh dumpxml Win7 | xpath -e /domain/devices/disk/source
Found 2 nodes in stdin:
-- NODE --
<source file="/var/lib/libvirt/images/Win7.img" />
-- NODE --
<source file="/var/lib/libvirt/images/Win7.iso" />

You may need to adapt /var/lib/libvirt/images/Win7.img in the following:

# virt-filesystems --long --parts --blkdevs -h -a /var/lib/libvirt/images/Win7.img
Name       Type       MBR  Size  Parent
/dev/sda1  partition  07   100M  /dev/sda
/dev/sda2  partition  07   32G   /dev/sda
/dev/sda   device     -    32G   -

Create your 64G disk:

# truncate -s 64G /var/lib/libvirt/images/outdisk

You'll need to expand /dev/sda2 (not the boot partition):

# virt-resize --expand /dev/sda2 /var/lib/libvirt/images/Win7.img /var/lib/libvirt/images/outdisk
Examining /var/lib/libvirt/images/Win7.img ...
 100% [progress bar] --:--
**********

Summary of changes:

/dev/sda1: This partition will be left alone.

/dev/sda2: This partition will be resized from 32G to 64G.  The 
    filesystem ntfs on /dev/sda2 will be expanded using the 
    'ntfsresize' method.

**********
Setting up initial partition table on outdisk ...
Copying /dev/sda1 ...
Copying /dev/sda2 ...
 100% [progress bar] 00:00
 100% [progress bar] 00:00
Expanding /dev/sda2 using the 'ntfsresize' method ...

Resize operation completed with no errors.  Before deleting the old 
disk, carefully check that the resized disk boots and works correctly.

Make a backup just in case (or use mv if you do not want the backup):

# cp /var/lib/libvirt/images/Win7.img /var/lib/libvirt/images/Win7.img.old
# mv /var/lib/libvirt/images/outdisk /var/lib/libvirt/images/Win7.img

Now boot !

For more info: man virt-resize

Seems the same question's already been asked on SuperUser - Where can I find Ubuntu Desktop releases on a vmdk

The following is the accepted answer:

I have not found any official ubuntu website distributing vmdk's.

According to http://www.vmware.com/appliances/directory/

very few suppliers of ubuntu desktop on vmdk exist (3 to be exact).

This seems to be focused on vm creation and distribution, and for now I consider this the de-facto website to go to for ubuntu desktop vmdk's: http://vmplanet.net/

These guys seem to have vm creation and distribution as a side-project to their main line of business: http://www.quotrader.com/
http://www.bitconstructor.com/

but the highest-voted answer is as follows:

You can find many VMs on VMware's site.

Open the site and go to "Virtual Appliances"; Ubuntu LTS 11.4 is one of the many images there.

The newest Ubuntu 11.04 VMware appliance starts out of the box with a desktop looking like Gnome 2.x, but can be transformed to start the Unity desktop from Ubuntu. Contrary to the basic Ubuntu 11.04 image this one has the latest VMware Tools installed.

Note: The VMware's site also seems to have Ubuntu 12.04, both desktop and server versions but it's the VMWare store and you have to buy it.

The OP also adds:

just make sure you have installed the latest vmware player so you don't have problems like I had with vmplayer not being able to run an virtual image made with a newer version.