I'd like to set the main font size (i.e. all text which isn't a heading or something else with an explicit font size) to a particular font size, in this case, 9.5pt.
I can't use the default package options. I also can't use \fontsize{9.5pt}{9.5pt}\selectfont at the start of my document because then latex (xelatex 3.1415926-2.4-0.9998 (MiKTeX 2.9)) selects some other font than the font set with \setmainfont.
What's a nice, simple way to set the default font size, analogous to the \setmainfont command from fontspec?
I have no version information about fontspec.

\setmainfont[Ligatures=TeX,Scale=0.95]{Some Nice Font}– egreg Oct 31 '13 at 21:11\documentclassand ending with\end{document}. Please also provide information about the version of XeLaTeX andfontspecthat you use. – Mico Oct 31 '13 at 21:11size10.clo, to use what you need. But it's not at all clear what you want. – egreg Oct 31 '13 at 21:15