Set maxlength for checkout postcode field in WooCommerce

Solution:

One way is to do it via jQuery, although the code you have already posted here works without any problems

function action_wp_footer() {
    // Returns true on the checkout page, when false, return
    if ( ! is_checkout() ) return;
    ?>
    <script>
        jQuery(document).ready(function($){
            $( '#billing_postcode' ).attr( 'maxlength', '5' );
            $( '#shipping_postcode' ).attr( 'maxlength', '5' );
        });
    </script>
    <?php
}
add_action( 'wp_footer', 'action_wp_footer' );