There was a video by Path Less Pedaled on Youtube where he said that 40mm tires did so the trick on cobblestone. But I would say in my opinion choices like 700x40mm or 27.5x2.00 are the more comfortable choices. Or I'd go minimum 38mm. By the way, the Vittoria Randonneur in 40c is actually 36mm (even the Panaracer Pasela 700x38 is 36mm).