Limit postal codes to 5 digits

functions.php

add_filter( 'woocommerce_default_address_fields' , 'custom_override_default_address_fields' );

// Our hooked in function - $address_fields is passed via the filter!
function custom_override_default_address_fields( $address_fields ) {
     $address_fields['postcode']['maxlength'] = 5;

     return $address_fields;
}