\documentclass[14pt]{report}
\usepackage{extsizes}
\usepackage[paperwidth=21cm,paperheight=21cm,left=0.2cm,right=0.2cm,top=0.5cm,bottom=0.5cm]{geometry}
\usepackage{pxfonts}
\pagestyle{empty}
\usepackage{graphicx}
\usepackage{booktabs}
\usepackage[table]{xcolor}
\definecolor{lightgray}{gray}{0.87}
\begin{document}
\begin{table}
  \begin{center}
  \rowcolors{2}{lightgray}{}
  \resizebox*{!}{\textwidth}{
    \begin{tabular}{lp{2.57cm}}
      \toprule
  Dataset     & . output  \\ 
      \midrule
  HERA1+2 NCep 820 & 66 / 70  \\ 
  HERA1+2 NCep 460 & 218 / 204  \\ 
  HERA1+2 CCep & 45 / 39  \\ 
  HERA1+2 NCem & 219 / 159  \\ 
  HERA1+2 CCem & 56 / 42  \\ 
  HERA1+2 NCep 575 & 219 / 254  \\ 
  HERA1+2 NCep 920 & 442 / 377  \\ 
  Correlated $\chi^2$  & 92  \\ 
  Log penalty $\chi^2$  & +6.1  \\ 
  \rowcolor{white}
      \midrule
  Total $\chi^2$ / dof  & 1363 / 1131  \\ 
  \rowcolor{white}
      \midrule
  $\chi^2$ p-value  & 0.00   \\ 
      \bottomrule
    \end{tabular}
  }
  \end{center}
\end{table}

\end{document}
