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