I think fractional expressions in my text is small and not normal?

Question

\documentclass[12pt]{book}  
\usepackage{amsmath,amsthm,amssymb,amsfonts}  
\usepackage{xepersian}  

\begin{document}  
\begin{persian}  
\lr{$-\frac{1}{3}$}  
\hspace{1cm}  
$-\frac{1}{3}$   
\hspace{1cm}   
\lr{1}  
\end{persian}  
\end{document}

Answer 1

If you want display-style (i.e., "large") fractions in inline or text-style math mode, as is the case in your MWE, use the macro \dfrac instead of \frac.

The macros \dfrac ("display-style \frac") and \tfrac ("text-style \frac") are provided by the amsmath package (which is loaded in your MWE).

Answer 2

If you want type fractions at normal size inline text, add \displaystyle between $ $ and before \frac, or use \dfrac (from amsmath package):

\documentclass[12pt]{book}
\usepackage{amsmath,amsthm,amssymb,amsfonts}  
\usepackage{xepersian}  
\begin{document}  
\begin{persian}  
\lr{$\displaystyle-\frac{1}{3}$}  
\hspace{1cm}  
$-\dfrac{1}{3}$   
\hspace{1cm}   
\lr{1}  
\end{persian}  
\end{document}

Answer 3

If \dfrac is too large, you also can have medium-sized maths with the nccmath package; in particular it defines an \mfrac command. Medium-sized maths are about 80 % if \displaystyle and most usually don't produce unequal interline spacing if you slightly increase \baselinestretch, e.g. setstretch{1.08} (it's the recommended way of doing; the command is defined in the setspace package.

You have a medsize and an mmmatrix environments, and a number of other medsize commands. An example for the fractions:

\documentclass[12pt, a4paper]{article} \usepackage[utf8x]{inputenc} \usepackage[T1]{fontenc} \usepackage{fourier} \usepackage{heuristica}

\usepackage{mathtools, nccmath}

\begin{document}

Here’s an \verb+align+ environment typeset first with \verb+\tfrac+, then \verb+\mfrac+, and finally \verb+\dfrac+:
\begin{align*}
      \rightarrow \quad &x=3, \tfrac{10}{3},\tfrac{11}{3}. \\
      \rightarrow \quad &x=3, \mfrac{10}{3},\mfrac{11}{3}. \\
      \rightarrow \quad &x=3, \frac{10}{3},\frac{11}{3}. \\
\end{align*}

This is a text in paragraph typeset with \verb+\tfrac+: $\textrm{C.S}=\{3,\frac{10}{3},\frac{11}{3}\}$.

This is a text in another paragraph typeset with \verb+\mfrac+: $\textrm{C.S}=\Bigl\{3,\mfrac{10}{3},\mfrac{11}{3}\Bigr\}$.

And finally a paragraph typeset with \verb+\dfrac+: $\textrm{C.S}=\biggl\{3,\dfrac{10}{3},\dfrac{11}{3}\biggr\}$.

\end{document}

It appears that nccmath may have some problems if used in a minipage environment, though I never had personally. See here and here. In case of problem, what could hopefully be a solution is sketched in the comments.