\tiny to \Huge) are determined. The classes are designed
on the assumption that they won’t be used with sizes other than the
set that LaTeX offers by default (10–12pt), but people regularly
find they need other sizes. The proper response to such a requirement
is to produce a new design for the document, but many people don’t
fancy doing that.
A simple solution is to use the extsizes bundle. This
bundle offers “extended” versions of the article, report, book and
letter classes, at sizes of 8, 9, 14, 17 and 20pt as well as the
standard 10–12pt. Since little has been done to these classes other
than to adjust font sizes and things directly related to them, they
may not be optimal — but they’re certainly practical.
More satisfactory are the KOMA-script classes, which are
designed to work properly with the class option files that come with
extsizes, and the memoir class that has its own
options for document font sizes 9pt–12pt, 14pt, 17pt, 20pt, 25pt,
30pt, 36pt, 48pt and 60pt.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=extsizes