Enforcing That \array Stays Square When Drawing Matrix

With tikz (There are others methods with tikz like matrix etc.)

\documentclass{article}
\usepackage{tikz}    

\begin{document} 

\begin{tikzpicture}
  \foreach \x in {1,2}
    \foreach \y in {1,2}
    {
      \draw (\x,\y) +(-.5,-.5) rectangle ++(.5,.5);
      \draw (\x,\y);
    }
    \node at (1,2) {$I_{r \times r}$} ;
    \node at (1,1) {0} ; \node at (2,1) {0} ;\node at (2,2) {0} ;
\end{tikzpicture}

\end{document}   

or you can do something like that but I'm not sure if all blank spaces are removed.

 \def\vh{\vrule height 0.6cm depth 0.4cm width 0 cm}% 
 \newcommand{\sq}[2][1cm]{\hbox to #1{\hfil\vh#2\hfil}}%   
 $
\begin{array}{@{}|@{}c@{}|@{}c@{}|@{}}
   \hline
   \sq{$I_{r \times r}$}&\sq{0}\\
   \hline
   \sq{0} & \sq{0}\\
   \hline
\end{array} 
$

the width/height of a cell is a parameter of the columntype C

\documentclass{article}
\usepackage{array}
\newcolumntype{C}[1]{@{}>{\rule[0.5\dimexpr-#1+1.2ex]{0pt}{#1}\hfil$}p{#1}<{$\hfil}@{}}
\begin{document}     
\Huge
$\begin{array}{|C{2cm} | C{2cm} |}\hline
  I_{r \times r} & 0 \\\hline
               0 & 0 \\\hline
\end{array} $ 
$\begin{array}{|C{2.5cm} | C{2.5cm} |}\hline
  I_{r \times r} & 0 \\\hline
               0 & 0 \\\hline
\end{array} $ 
\end{document}