Actually, after a little googling myself, sounds like it's the free size that does it, not the actual disk size.
If you don't want to screw with partitions and want to try it out, this should do it:
Make a shortcut on your desktop to the command 'cmd'. Right click and run it as administrator so you can use the following commands.
c:
cd \
fsutil file createnew foo 1200000000000
You will need to keep the huge file until you either change the scratch disk or get jiggy with the partitioning software to make a scratch disk partition that's smaller, but I believe this will at least get you going with a minimum of craziness. Remember if you fix the partitions or use a USB scratch or whatever, there's a huge file in c:\ that can disappear...