The height of the header is determined by this CSS selector:
The #Header a is nested in the #Header. It provides a link to the homepage,
and sets the header image background.
You find this code in DarkGrey.css that you added in step 9 on page 222. For the header to get its size, you need an <a /> element in the #Header div. You added this anchor element earlier. The code example of step 6 on page 204 shows you how it should look like.
Hope this helps,