You'll have to forgive me, as I haven't played with Wubi recently enough to be particularly experienced with it.
There does seem to be a guide on this
on the Ubuntu forums here. The process might be a little bit involved (you may need an Ubuntu Forums account to download the script, not that we're sending you away by any means!), but that script does appear to create a copy of your virtual disks, while making them larger at the same time. You then swap the 'new' larger disk for the old one by renaming them, and you should have more space when you reboot back into the Wubi Ubuntu install.
As I say, I'm no Wubi expert, so it's quite possible someone else will think of a better way to do this.
If you need any specific help actually running through those commands, or run into trouble, we can try and help go through each step.
Let us know how you get on! :)