Create a custom length excerpt.

wp_excerpt_length.php

<?php
/**
 *  Create a custom length excerpt.
 *
 *  @param   integer  $limit    How many words you want returned
 *  @return  string   $content  The content string
 */
function wp_excerpt_length( $limit=50 ) {
    $content = get_the_content();

    $content = preg_replace( '/\[.+\]/', '', $content );
    $content = apply_filters( 'the_content', $content );
    $content = str_replace( ']]>', ']]&gt;', $content );
    $content = strip_tags( $content );

    $content = explode( ' ', $content, $limit );

    if( count( $content) >= $limit ) {
        array_pop( $content );
        $content = implode( ' ', $content ) . '...';
    } else {
        $content = implode( ' ', $content );
    }

    return $content;
}