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.
Pragmatists, therefore, will tend to go for the classes in 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 won't be optimal - but they're certainly practical.