How can i limit the character length in excerpt? [duplicate]

Possible Duplicate:
excerpt in characters

I have a question after reading this post (How to highlight search terms without plugin). I like this function(Search Term Without Plugin) very much but the character length is too long. What php code should i add to make the excerpt shorter? Would appreciate if someone can suggest it. Thank you!

add these lines in function.php file

function custom_excerpt_length( $length ) {
        return 20;
    }
    add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );