A home office can be as small as a folding file or as large as a room. If you're running a business out of your home, you'll obviously need much more space and equipment than a family looking for a corner to file papers and tend to correspondence.
The important thing to remember about creating a home office is to make it suit your family's needs. For example, if your children need to use a computer, you'll want to set up space that can be shared -- not a corner in your bedroom. Likewise, equipment and supplies should be selected for versatility.