Two “the_excerpt” questions

You need to hook into two WordPress filters.

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

http://codex.wordpress.org/Plugin_API/Filter_Reference/excerpt_length

function custom_excerpt_more( $more ) {
    return '[.....]';
}
add_filter( 'excerpt_more', 'custom_excerpt_more' );

http://codex.wordpress.org/Plugin_API/Filter_Reference/excerpt_more