The following code from a previous answer to one of my questions displays Woocommerce “Size” product attributes below each product on the shop page:

add_action( 'woocommerce_after_shop_loop_item_title', 'display_size_attribute', 5 );
function display_size_attribute() {
    global $product;

    if ( $product->is_type('variable') ) {
        $taxonomy = 'pa_size';
        echo '<span class="attribute-size">' . $product->get_attribute($taxonomy) . '</span>';
    }
}

How do I change the code to only display “in-stock” items (of product attribute “Size”)?