Sum of orders per customer ID
Applying a JOIN sounds pretty easy

SELECT article1, article2, size, order_date, SUM(quantity)
FROM orders 
INNER JOIN customers ON orders.order_no = customers.order_no
WHERE customers.customer_no = '1027'
GROUP BY article1, article2, size, order_date, SUM(quantity)


You should note that nested SELECT statements in a WHERE clause almost always lead to bad performance.



Also that kind of unnormalized data model looks weird, but there may be good reasons why to keep order_no and customer_no pairs in a single association table. May be its just bad naming, something like customers_orders might probably have been a better choice.