1

I have a book with the font size of equation smaller than that of text. Please look at how small $\pi$ compared to the rest:

\documentclass[openany]{book}
\usepackage{amsbsy}
\usepackage{changepage}
\usepackage{geometry}
\geometry{paperheight=9.8125in,paperwidth=8in, left=.5in, 
right=.5in,top=.75in,bottom=.4375in }
\usepackage{fancyhdr}
\fancypagestyle{plain}{\fancyhf{} \fancyhead[EL,OR]{\textbf 
{\thepage}}\renewcommand{\headrulewidth}{0pt}}
\pagestyle{plain}
\renewcommand{\rmdefault}{pag}
\usepackage[utf8]{inputenc}
\usepackage[hyphens]{url}
\usepackage[authoryear,round]{natbib}
\newcommand{\titlefontarash}{\usefont{OT1}{pag}{b}{n}\fontsize{25} 
{\baselineskip} \selectfont}
\newcommand{\basefontarash}{\usefont{OT1}{pag}{b}{n}\fontsize{18} 
{\baselineskip} \selectfont}
\newcommand{\datefontarash}{\usefont{OT1}{pag}{m}{n}\fontsize{12} 
{\baselineskip} \selectfont}
\begin{document}
\title{\titlefontarash  respect}
\author{\basefontarash good boy}
\date{\datefontarash April 21, 2019}
{ 
\maketitle
\let\cleardoublepage\relax
\frontmatter
} 
\vspace*{\fill}
\begin{center}
\basefontarash To beauty...
\linebreak 
\linebreak
\basefontarash To nice...
\end{center}
\vspace*{\fill}
\tableofcontents
\begin{adjustwidth}{2.25in}{0pt}
\mainmatter
{Introduction}
{\chapter{beauty}
fish (\citet{fox(1994)})  number $\pi +\pi +\pi +v\pi +\pi +v\pi +\pi +\pi 
+\pi +\pi +v\pi +v\pi +\pi +\pi +\pi +\pi +\pi +\pi +\pi +\pi +$ fish 
\textbf{RIVER ${\boldsymbol{\pi+\pi+\pi+\pi+\pi+\pi+\pi+\pi+\pi}}$} 
following .
 \section{fox1}
\subsection{fox2}
\subsection{fox3}
\subsection{fox4}
\subsection{fox5}
\subsection{fox6}
\subsection{fox7}
 Now we are fox...
Let us see what happens
\section{fish}
fish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catvfish fox dog catvfish fox dog catfish fox dog catfish fox dog catfish 
fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox 
dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish 
fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catfish fox dog cat
}
 \section{fish5}
fish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catvvfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish 
fox dog cat
\section{dog}
fish fox dog catvvvfish fox dog catfish fox dog catfish fox dog catfish fox 
dog catfish fox dog cat
\subsection{dog  ${\boldsymbol{\pi}}$}
 \section{mouse}
 \section{milk}
\chapter{pishi}
\end{adjustwidth}
\begin{thebibliography}{9}
\bibitem[fox(1994)]{fox(1994)}fox, T. L. (1994) \textit{fox.} New York: fox.  

\end{thebibliography}
 \end{document}

Do you know how to change the fontsize of equation for the entire book in one command line not to do it for every equation?

0
2

I do not think you need to change the fontsize. The issue here is that your used letter \pi is simply a small letter. Add for example the following (not meaningful!) sequence to your pis: + \sqrt{145} \Omega +v\pi + a^2 +2ab + b^2 + you get a better overview about the resulting mathematical printing:

\documentclass[openany]{book}

\usepackage{amsbsy}
\usepackage{changepage}
\usepackage{geometry}
\geometry{paperheight=9.8125in,paperwidth=8in, left=.5in, 
right=.5in,top=.75in,bottom=.4375in }

\usepackage{fancyhdr}
\fancypagestyle{plain}{\fancyhf{} \fancyhead[EL,OR]{\textbf 
{\thepage}}\renewcommand{\headrulewidth}{0pt}}
\pagestyle{plain}

\renewcommand{\rmdefault}{pag}

\usepackage[utf8]{inputenc}
\usepackage[hyphens]{url}
\usepackage[authoryear,round]{natbib}
\newcommand{\titlefontarash}{%
  \usefont{OT1}{pag}{b}{n}\fontsize{25}{30} \selectfont% <============== 30
}
\newcommand{\basefontarash}{\usefont{OT1}{pag}{b}{n}\fontsize{18} 
{\baselineskip} \selectfont}
\newcommand{\datefontarash}{\usefont{OT1}{pag}{m}{n}\fontsize{12} 
{\baselineskip} \selectfont}


\begin{document}

\title{\titlefontarash  respect respect respect respect respect respect 
  respect respect respect respect respect respect respect respect respect 
  respect respect respect respect respect respect respect respect }
\author{\basefontarash good boy}
\date{\datefontarash April 21, 2019}
{ 
\maketitle
\let\cleardoublepage\relax
\frontmatter
} 
\vspace*{\fill}
\begin{center}
\basefontarash To beauty...
\linebreak 
\linebreak
\basefontarash To nice...
\end{center}
\vspace*{\fill}
\tableofcontents

\begin{adjustwidth}{2.25in}{0pt}
\mainmatter
{Introduction} % <================================================ ?????

\chapter{beauty}
fish (\citet{fox(1994)})  number $\pi +15 \pi + \sqrt{145} \Omega +v\pi + a^2 +2ab + b^2 + \pi +v\pi +\pi +\pi 
+\pi +\pi +v\pi +v\pi +\pi +\pi +\pi +\pi +\pi +\pi +\pi +\pi +$ fish 
\textbf{RIVER ${\boldsymbol{\pi+\pi+\pi+\pi+\pi+\pi+\pi+\pi+\pi}}$} 
following .
 \section{fox1}
\subsection{fox2}
\subsection{fox3}
\subsection{fox4}
\subsection{fox5}
\subsection{fox6}
\subsection{fox7}
 Now we are fox...
Let us see what happens
\section{fish}
fish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catvfish fox dog catvfish fox dog catfish fox dog catfish fox dog catfish 
fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox 
dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish 
fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catfish fox dog cat

 \section{fish5}
fish fox dog catfish fox dog catfish fox dog catfish fox dog catfish fox dog 
catvvfish fox dog catfish fox dog catfish fox dog catfish fox dog catfish 
fox dog cat
\section{dog}
fish fox dog catvvvfish fox dog catfish fox dog catfish fox dog catfish fox 
dog catfish fox dog cat
\subsection{dog  ${\boldsymbol{\pi}}$}
 \section{mouse}
 \section{milk}
\chapter{pishi}
\end{adjustwidth}
\begin{thebibliography}{9}
\bibitem[fox(1994)]{fox(1994)}fox, T. L. (1994) \textit{fox.} New York: fox.  

\end{thebibliography}
 \end{document}

with the resulting page:

resulting mathematic

Please see that I changed your definition of

\usefont{OT1}{pag}{b}{n}\fontsize{25}{30} \selectfont% <============== 30

to use 30 instead \baselineskip which gives you a better result, if your title is longer than one line (see, I added some more word to the relevant title line for testing).

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