
\documentclass{article}
  \usepackage{graphicx}
  \pagestyle{empty}

\begin{document}
\resizebox{6cm}{!}{
    $\sum_a^b$
    \begin{minipage}[b]{2.5ex}\[\sum_a^b\]\end{minipage}
    $\displaystyle \sum_a^b$
    $\displaystyle \sum\nolimits_a^b$
    $\sum\limits_a^b$
}
\end{document}
