Wordpress - Display single product attribute value on Shop page (Woocommerce)

Just use global $product then use get_attribute() method of that product object, like below-

$size = $product->get_attribute( 'pa_size' );

And you can also get that by below code-

global $product;
$size = array_shift( wc_get_product_terms( $product->id, 'pa_size', array( 'fields' => 'names' ) ) );

Rememeber you need to use must the global $product.