The LaTeX standard classes have a concept of a (base) "document
font" size; this size is the basis on which other font sizes (those
from \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, 14pt and 17pt.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=extsizes