\documentstyle{article}
\font\frtnex=cmex10 scaled\magstep2
\def\LargeMath{\Large\textfont3\frtnex}
\begin{document}

\begin{displaymath}              
\sum_{k=0}^{N}
\end{displaymath}

\begin{displaymath}              
\sum_{k=0}^{N}
\end{displaymath}

{\Large
\begin{displaymath}              
\sum_{k=0}^{N}
\end{displaymath}
}

{\LargeMath
\begin{displaymath}              
\sum_{k=0}^{N}
\end{displaymath}
}

{\makeatletter\@setsize\Cheese{22pt}\xviipt\@xviipt\makeatother
\begin{displaymath}              
\sum_{k=0}^{N}
\end{displaymath}
}

\end{document}
