A clean install with a fresh reformat takes about 8 GB before updates.
But if you don't reformat when you do the clean install, it will e that 8 GB plus however much space your previous installation took. But you can easily clean that up by running Disk Cleanup as an admin once you've got your files copied back over - thanks to Jessica for making sense of my original post.
You can perform a clean installation using an upgrade disc, here's how:
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It's a rather time consuming process, and if your not confortable working with the registry, you may want to purchase the "full" version of Windows 7. Of course you could always still upgrade, but there's a 95% chance you'll come across a lot of problems upon completion.