Fraction with longer line

Here's an extended version of \frac, supporting an optional argument for a wider line:

\documentclass{article}
\DeclareRobustCommand{\frac}[3][0pt]{%
  {\begingroup\hspace{#1}#2\hspace{#1}\endgroup\over\hspace{#1}#3\hspace{#1}}}
\begin{document}
\[
  \frac{x}{y} \quad \frac[5pt]{x}{y}
\]
\end{document}

Of course you can define your own frac command similar instead of redefining the existing \frac, also with a default value bigger than 0pt.

The definition is similar to the definition in amsmath.sty, except that file is using \@@over.

Or, you could define your own macro using the original \frac:

\DeclareRobustCommand{\widefrac}[3][5pt]{%
  \frac{\hspace{#1}#2\hspace{#1}}{\hspace{#1}#3\hspace{#1}}}
...
\widefrac{x}{y} ... \widefrac[8pt]{x+y}{y}

In case you really want fractions with a longer line, you can just pad both numerator and denominator with spaces. (Padding the longer of the two would acually suffice.) \frac{\ a\ }{\ b\ } gives

For a longer line use, e.g., \quad instead of \ .

\newcommand\bigfrac[2]{%
  \begin{array}{c}
    #1 \\
    \hline
    #2
  \end{array}}