Yes they are discriminating. No they should not be forced to provide all sizes, as that would be an unwarranted interference by the nanny state into the operation of market forces. Someone could sue them, but such an action would fail because there is no contractual or legal obligation on any shop to provide any particular item. It would be like suing a butcher's shop for not selling chocolate biscuits. A contract between a shop and a customer to sell something can only be done with the agreement of both parties; it would be wrong for a customer to force a shop to sell a particular item, just as it would be wrong for a shop to force a customer to buy something.