Change font size of one equation in align environment

Question

\begin{align}
    \label{e1}
    a+b&=c\\
    \label{e2}
    a+b+c+d+e+f+g+h+i+j+k+l&=1+2+3+4+5+6+7+8+9\\
    \label{e3}
    c+d&=e
\end{align}

From this page I am able to reduce the font size of all equations. But, how to change the font size of only one equation (e2).

Answer 1

Another possibility would be the \medmath command from nccmath, which sets the math fontsize between \scriptstyle and \displaystyle (~ 80 % of \displaystyle):

\documentclass{article}
\usepackage{mathtools, nccmath}

\begin{document}

\begin{align}
\label{e1}
                a+b & = c \\
    \label{e2}
\medmath{a+b+c+d+e+f+ g+h+i+j+k+l} & = \medmath{ 1+2+3+4+ 5+6+7+8+9}\\
    \label{e3}
                c+d & = e
\end{align}

\end{document} 

A last possibility in some cases, if you don't use margin notes, consists in loading the geometry package, which produces more decent margins and thus slightly increases text width.

Answer 2

\documentclass{article}
\usepackage{mathtools}


\begin{document}
better is reorganise your equations:
\begin{align}
\label{e1}
                a+b & = c                   \\
    \label{e2}
    \begin{multlined}[b]a+b+c+d+e+f+\\
                        g+h+i+j+k+l
    \end{multlined} & = \begin{multlined}[t]1+2+3+4+\\
                                            5+6+7+8+9
                        \end{multlined}     \\
    \label{e3}
                c+d & = e
\end{align}
or 
\begin{align}
\label{e1}
a+b & = c                   \\
\label{e2}
A   & = B    \\
\label{e3}
c+d & = e
\end{align}
where are
\begin{align}
A & = a+b+c+d+e+f+g+h+i+j+k+l   \label{eq:A}    \\
B & = 1+2+3+4+5+6+7+8+9         \label{eq:B}
\end{align}

\end{document}