About how large is a clean install of OS X?
The Install OS X Mountain Lion.app I have is about 4 GB, but the uncompressed size shown by Pacifist is about 9 GB. My initial snapshot of a 10.8 VM was about 11 GB, but about 1 GB was taken up by /var/vm/swapfile.

If you have a laptop with 4 GiB of RAM, /var/vm/sleepimage will take up about 4 GiB after you put the laptop to sleep once.