Extending my comment: Simply redefining \normalsize
is not a good idea. There are further lengths which are chosen matching the base font size, specifically regarding vertical spacing. For example, if you look into the file book.clo
, which contains class options for book
, you can see it.
For a consistent change I recommend changing the document class.
the extsizes bundle provides classes similar to standard classes offering a base font size from 9pt to 20pt.
memoir
can also be used with 20pt and more. In this regard, the option extrafontsizes
may be useful. For very high font size a scalable font should be used.
KOMA-Script classes, such as scrartcl
, scrbook
and scrreprt
support arbitrary base font sizes. You could even user other units than pt to specify the size.
Since KOMA-Script classes offer many more features, this would be my choice.
The further way depends on how your're producing the footer. On chapter starting pages, the page style is plain
.
With default page styles the base font size should be used, so there's no problem.
With fancyhdr
, you could simply add \small
, \normalsize
or any orher font size command to the beginning of your header and footer macros. If necessary, use \fancypagestyle{plain}{\fancyhf{}\cfoot{\small\thepage}}
, like Ulrike already said in comments to question and to this answer.
With a KOMA-Script class, I recommend to use scrpage2
instead of fancyhdr
. Since it belongs to the KOMA-Script bundle, it's very well integrated with the classes. That should also solve the problem. If necessary, use a font size command such as \small
, \tiny
or just \normalsize
within header and footer commands. Note: for plain
pages do it also within the optional argument, such as \cfoot[\tiny\thepage]{\tiny\thepage}
. scrpage2
is explained in the KOMA-Script manual.
With KOMA-Script there are more and even easier possible solutions. You could use \addtokomafont
or \setkomafont
for specifying the font size. For example:
\addtokomafont{pagenumber}{\tiny}
Further font option names, which you might use in this regard: pagehead
, pageheadfoot
, pagefoot
.