The basic font sizes of the standard document classes are built into the classes themselves. The extsizes classes define a number of parallel classes to the standard ones with a variety of sizes, including 8 pt.
\documentclass[8pt]{extarticle} % or {scrartcl} (Koma class)
\usepackage{fontspec}
\usepackage{lipsum} % for random text
\setmainfont{Linux Libertine}
\begin{document}
\section{A section}
\subsection{A subsection}
\lipsum
\end{document}
The since all sizes of other text (such as \Large, \footnotesize etc.) are defined relative to the document class size, it's better to use this sort of solution, if the normal document classes work for you.
The memoir class also allows a 9pt option, but not 8pt.
The koma document classes also allow specifying an arbitrary point size.
