Use the following PHP function to limit the number of characters ($limit) to be returned from the original string ($source).

//limit text
function limit_text($limit, $source = null){
$excerpt = $source == "content" ? get_the_content() : get_the_excerpt();
$excerpt = preg_replace(" (\[.*?\])",'',$excerpt);
$excerpt = strip_shortcodes($excerpt);
$excerpt = strip_tags($excerpt);
$excerpt = substr($excerpt, 0, $limit);
$excerpt = substr($excerpt, 0, strripos($excerpt, " "));
$excerpt = trim(preg_replace( '/\s+/', ' ', $excerpt));
$excerpt = $excerpt.'...';
return $excerpt;
}