I think you are right. Experts say that Brown Rat (Rattus norvegicus) droppings are 17-20 mm long, although size varies greatly. The colour varies also with food content, and these look like fresh droppings, not yet dehydrated. These seem to be 20-25 mm long, so a large specimen may have produced them. Brown Rats drop faeces in groups in corners or along the walls. Black Rats (R. rattus) leave their droppings singly and at random all over the place. Their droppings are smaller and proportionally thinner. So I would go for brown rat in this case.