After numerous tries with various approaches, I finally managed to end up with a 40 Gig virtual machine to host Windows.
The procedure that worked for me was to
- clone the existing hard drive using the function in the GUI of Virtual Box (and not the command line). VB created a new folder and put the new virtual machine file into that folder.
- go into the command line and make the virtual file 40 gigs in size
sudo VBoxManage modifyhd newVirtualMachine.vdi --resize 40960
- start up virtual box
- select the new virtual machine
- click on settings, click on disk, right click, choose "Remove"
- still on settings, right
- in Windows, start Windows Computer Management
- go to Disk/Storage management
- right click on existing small partition, select "extend"
- click OK in dialog box

When I tried using the command line to clone the virtual drive, I got the file, but the GUI in Virtual Box didn't see it.
I tried a couple of other partition managers before stumbling on the Windows Computer Management and either I couldn't get them to install properly or I couldn't get them to extend the smaller partition to a bigger one.

The most helpful website I found was
http://tips.kaali.co.uk/2012/03/16/expand-or-increase-the-size-of-virtual-box-vdi-dis/ though I did not create the hard drive and then clone the existing drive to the new drive as that page says, rather I cloned the existing drive and then resized it to 40G.

Finally, when I started windows in the new virtual machine, I got a "this copy of windows may not be authentic" warning. I followed the link on the warning and that "authenticated" my copy.