J Biddle's response does not address your concerns, even though a sincere try. Both Doug Robbins and Stefan Blom are addressing your concerns of reducing how much space is allocated to the header and footer.
Here is my Header/Footer Settings Recap. You may not need all that information, though.
A key idea is that each Word document reserves room for a header and a footer and lets you position that space from the top and bottom of the page. If that reserved room is not used, it still is not available in the body of the document. It can be a little or a lot of space.
However, if you place material in the header or footer that exceeds that reserved space, it will use that additional space, taking it away from the room available in the body.
It is also possible to place images behind text (watermarks) that do not take away from the space available in the body. Likewise, it is possible to place shapes or frames in a header or footer that act to move the left and right margin.
The space after paragraph setting of the Heading Style and the space before paragraph setting of the Footer Style can also change how much space is available in the body of the document.