functions.php

function my_wc_custom_shipping_fields( $fields ) {
  $fields['shipping_postcode']['maxlength'] = 5;

  return $fields;
}

add_filter( 'woocommerce_shipping_fields', 'my_wc_custom_shipping_fields' );

function my_wc_custom_billing_fields( $fields ) {
  $fields['billing_postcode']['maxlength'] = 5;

  return $fields;
}

add_filter( 'woocommerce_billing_fields', 'my_wc_custom_billing_fields' );