The LaTeX standard classes have a concept of a (base) “document
font” size; this size is the basis on which other font sizes (those
\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 are at least 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. The classes also offer size setup for any old font size, and the scrextend package can extend this facility for use with any class:
will indeed set up the main document font to have size
with an appropriate default baselineskip. The package “knows” about
KOMA-script's default sizes, and for eccentric sizes
such as the example, it will produce a warning:
Using fallback calculation to setup font sizes
(users should avoid becoming excited about that…). The package suffers from the same problem as does extsizes: the resulting font sizes are the only feature of the document that is changed, and the appearance of the resulting document will probably not be as good as if the document class had been designed for use at the size chosen.
Many classes, designed to produce typeset results other than on “ordinary” paper, will have their own font size mechanisms and ranges of sizes. This is true, for example, of poster classes (such as a0poster), and of presentation and lecturing classes (such as beamer).