Easily change behavior of \DeclarePairedDelimiter

I continue to believe that one should use \left and \right where really needed and any sort of automatism for this is bad.

However, here's a simple and working implementation of your macro, that I call \xDeclarePairedDelimiter

\documentclass{scrartcl}
\usepackage{mleftright,xparse}

\NewDocumentCommand\xDeclarePairedDelimiter{mmm}
 {%
  \NewDocumentCommand#1{som}{%
   \IfNoValueTF{##2}
    {\IfBooleanTF{##1}{#2##3#3}{\mleft#2##3\mright#3}}
    {\mathopen{##2#2}##3\mathclose{##2#3}}%
  }%
 }
\xDeclarePairedDelimiter{\set}{\lbrace}{\rbrace}

\begin{document}

\begin{align*}
&\liminf \set{\sum}\\
&\liminf \set*{\sum}\\
&\liminf \set[\big]{\sum}\\
&\liminf \set[\Big]{\sum}
\end{align*}

\end{document}