The closest we have to a barrel standard is the "Bordeaux Barrel" or barrique that holds 225 liters, or 300 normal sized bottles of wine. This barrel is almost a meter long, and about 69 cm in diameter, which stands in rather stark contrast to the barrel Gérard Bru of Château Peuch Haut in the Languedoc region of southern France announced this week. He had a local carpenter build him a barrel that is 12 meters long, six meters in diameter and weighs more than 40,000 kilos empty. And it holds 300,000 liters of wine, which is 400,000 bottles. That's a lot of wine.
Bru says he won't fill the barrel though - he wants to make an event space out of it and hold tastings and parties inside it. So now we will have toasted guests who are themselves barrel fermented, after a fashion.