[XeTeX] selecting font size

[Brian Wilson]

Thank you to Peter and Fr. Michael for pointing out the NFSS2.

A google search gave me the following wiki article which mentions how to set
the font for a non-XeTeX engine.

http://mactex-wiki.tug.org/wiki/index.php?title=Font_Selection

For \documentclass{book} I can choose between 10pt-12pt. This is too small
for booksfor the visually impared or books in languages whose font size is
natively small. Thai fonts are really small. A 16pt Thai font is about 11pt
for an English font. A 10pt Thai font is about 7pt in English.

There is a disconnect and I know I am missing something simple, but can't
find the documentation on this. The pdf for NFSS2 was not clear on how to
set the fontsize globally. My failed attempt follows.

Any advice is appreciated.
Brian Wilson

\documentclass[a5paper,12pt,draft]{book}
\special{papersize=148mm, 210mm}

%%%%%%%%%%%%%%%%%%%%%%%%
% Packages
\usepackage{fontspec}% provides font selecting commands
\usepackage{xunicode}% provides unicode character macros
\usepackage{xltxtra} % provides some fixes/extras

%%%%%%%%%%%%%%%%%%%%%%%%%
% Font Definitions
\setmainfont{Saysettha MX}
\fontsize{18}{25}
\selectfont

\begin{document}

ຖ້າເອົາໃຈໃສຕໍ່ຂໍ້ຄຳສັ່ງທຸກຢ່າງຂອງພະອົງຂ້າພະອົງກໍຈະບໍ່ໄດ້ຖືກອັບອາຍຂາຍໜ້າ.
ເມື່ອຮຽນຮູ້ການຕັດສິນອັນຊອບທຳຂອງພະອົງຂ້າພະອົງຈະຍົກຍໍພະອົງດ້ວຍຈິດໃຈຜຸດຜ່ອງ.

\end{document}
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