Expanding the disk itself is very easy.
1) Open a Terminal
2) cd to the directory with the .vmdk you wish to expand.
3) Run: "/Applications/VMware\ Fusion.app/Contents/MacOS/diskTool -X 50GB <file.vmdk>" where 50GB is the size you want and file.vmdk is the correct name for your VM.
4) Wait.
Now, the issue you run into is that you're increasing the physical (virtual) drive size, but the guest OS doesn't know about this. Just as if you had a 100GB drive, formatted it, installed Windows, and then somehow magically increased the disk to 120GB -- the formatting/partitioning is still set for 100GB.
What this means is that you'll have a larger drive, but the Windows install will only be configured for the old size. You must expand the partition within the VM. You can do this via something like Partition Magic or Acronis Disk Suite. I recently used Gparted (free) to expand a partition in a VM and it worked fine.
Mike