How to set font size of footnotes correctly in memoir?

Question

I am type setting a book with the memoir class and have to follow the rules that are given by the publishing house. For that rules I have to define the exact font sizes: The font size of the footnotes should be 8.5pt with and line space 10.5pt, so I tried:

\renewcommand{\footnotesize}{\fontsize{8.5pt}{10.5pt}}

But I get some strange results: The font size seems to be much bigger than 8.5pt. When I try

\renewcommand{\footnotesize}{\small}

The font occurs smaller although \small means 9pt according the the memoir manual. What am I doing wrong?

\documentclass[a4paper,10pt,twoside]{memoir}

\usepackage{polyglossia} 
\usepackage{blindtext}
\usepackage{fontspec}


\makeatletter
\def\footnoterule{\kern-8\p@
  \hrule \@width 2in \kern 7.6\p@} % the \hrule is .4pt high
\makeatother

\renewcommand{\footnotesize}{\fontsize{8.5pt}{10.5pt}}

\footmarkstyle{\hfill\textsuperscript{#1} }

\setlength{\footmarkwidth}{0.6cm}
\setlength{\footmarksep}{-0.6cm}
\setlength{\footnotesep}{.65\onelineskip}
\setlength{\skip\footins}{1.6\onelineskip}

\begin{document}
\tableofcontents*
\chapter{Test, nothing else}
\section{Don't wast you time reading this!}
\Blindtext
\footnote{\blindtext}
\blindtext
\section{Test124}
\Blindtext
\chapter{Test2}
\Blindtext \Blindtext \Blindtext
\end{document}

Answer 1

You need to activate the new font size with \selectfont like this \renewcommand{\footnotesize}{\fontsize{8.5pt}{10.5pt}\selectfont}