\documentclass[12pt,a4paper]{book}
\usepackage{amsmath,amssymb,amsthm,color}
\usepackage{xepersian}


\newcommand{\BigFig}[1]{\parbox{0.1pt}{\large #1}}
\newcommand{\BigZero}{\BigFig{0}}

\begin{document}

$$x+\BigZero =y$$

\end{document}