I think my theme is using this function to generate excerpts. But I couldn’t find how to increase its length.
if (get_option($shortname.'_use_excerpt') == 'on' && $postExcerpt <> '') {
if ($echo) echo $postExcerpt;
else return $postExcerpt;
} else {
$truncate = $post->post_content;
$truncate = preg_replace('@\[caption[^\]]*?\].*?\[\/caption]@si', '', $truncate);
if ( strlen($truncate) <= $amount ) $echo_out = ''; else $echo_out = '...';
$truncate = apply_filters('the_content', $truncate);
$truncate = preg_replace('@<script[^>]*?>.*?</script>@si', '', $truncate);
$truncate = preg_replace('@<style[^>]*?>.*?</style>@si', '', $truncate);
$truncate = strip_tags($truncate);
if ($echo_out == '...') $truncate = substr($truncate, 0, strrpos(substr($truncate, 0, $amount), ' '));
else $truncate = substr($truncate, 0, $amount);
if ($echo) echo $truncate,$echo_out;
else return ($truncate . $echo_out);
};