Specific font for specific characters with mathspec package

Question

I am trying to manipulate the font, font size and thickness of the mathematical characters individually. I came into this solution at stackexchange; XeTeX/mathspec punctuation issue

...
\makeatletter
\ernewcommand\eu@MathPunctuation@symfont{Latin:m:n}
\DeclareMathSymbol{,}{\mathpunct}{\eu@MathPunctuation@symfont}{`,}
...

To make the +-= bold and plain other I tried this way (which did not work);

\documentclass[9pt]{article}
\usepackage{extsizes}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathspec}


\setmathfont(Digits,Latin)[Numbers={Lining,Proportional},Lowercase={Regular},Uppercase={Regular}]{Arial}
\setmathfont(Greek)[Scale=1.15, Lowercase={Regular},Uppercase={Regular}]{Symbol}
\setmainfont[Ligatures=TeX]{Arial}

\makeatletter
\ernewcommand\eu@MathPunctuation@symfont{Latin:bx:n}
\DeclareMathSymbol{+}{\mathpunct}{\eu@MathPunctuation@symfont}{`+}
\DeclareMathSymbol{-}{\mathord}{\eu@MathPunctuation@symfont}{`-}
\DeclareMathSymbol{=}{\mathrel}{\eu@MathPunctuation@symfont}{`=}
\makeatother


\begin{document}
\thispagestyle{empty}

$+-=$+-=

\end{document}

Question 1) How can we make only characters "+-=" bold and other characters plain. Question 2) Can we set the font size of the "a" in the expression "x^a". I want my normal text to be 9pt and sub, sup, frac and frac{frac} characters to be fixed at 8pt not shrinking as the level increases. Any help is appreciated. Thank you all reading or responding.

Answer 1

You're forgetting to declare the suitable math symbol font; you have also to use \XeTeXDeclareMathSymbol if you want a real minus sign.

\documentclass[9pt]{article}
\usepackage{extsizes}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathspec}


\setmathfont(Digits,Latin)[
  Numbers={Lining,Proportional},
  Lowercase=Regular,
  Uppercase=Regular
]{Arial}
\setmathfont(Greek)[
  Scale=1.15,
  Lowercase=Regular,
  Uppercase=Regular
]{Symbol}
\setmainfont[Ligatures=TeX]{Arial}

% subscripts are 8pt at any level
\DeclareMathSizes{9}{9}{8}{8}

\makeatletter
\DeclareSymbolFont{Latin:bx:n}{EU1}{\eu@Latinmathsfont}{bx}{n}
\ernewcommand\eu@MathPunctuation@symfont{Latin:bx:n}
\DeclareMathSymbol{+}{\mathbin}{\eu@MathPunctuation@symfont}{`+}
\DeclareMathSymbol{=}{\mathrel}{\eu@MathPunctuation@symfont}{`=}
% fight against amsmath using the mathcode of `-`
\AtBeginDocument{%
  \XeTeXDeclareMathSymbol{-}{\mathbin}{\eu@MathPunctuation@symfont}{"2212}%
}
\makeatother


\begin{document}
\thispagestyle{empty}

$+-=$+-=

$a^{a^a}$

\end{document}