How can I use Windows via Parallels or Fusion with MINIMAL HARD DISK space?
Both Fusion and Parallels give you the option of creating dynamically sized virtual hard drives. They allow you to specify the maximum size of the virtual hard drive, but do not actually consume (or reserve) that much disk space on the real hard drive. So to answer your question, just be sure to create a dynamically sized virtual hard drive. (I believe this is the default in both Fusion and Parallels.)