What is the largest barrel size?

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Dear Dr. Vinny,
What is the largest barrel size? Is it a hogshead?
—Jason, via the Internet
Actually, the world's largest wine barrel is considered to be the Heidelberg Tun, contained within the cellars of Germany's Heidelberg Castle. It was built in 1751 from 130 oak trees by Prince Elector Karl Theodor, and it stands 7 meters high, 8 1/2 meters wide, holds 220,000 liters (that's more than 58,000 gallons) of wine, and has a dance floor built on top of it. Legend has it that there was an 18th-century dwarf named Perkeo who served as court jester and keeper of the royal wine keg. He's said to have died when he mistakenly drank a glass of water. I couldn't make this stuff up if I tried!
You're correct that the hogshead barrel (300 liters/79 gallons) is one of the larger standard sizes out there. But I spoke with a sales representative at Seguin Moreau cooperage who says that they regularly make 700 liter barrels, which tend to be popular in the Rhône.
There are containers made of oak that are larger than this, but they aren't considered "barrels" anymore; rather, they're "tanks" or "vats" and are used more frequently for Cognac than for wine. The rep tried to sell me one that holds 600,000 liters (but without a dwarf or a dance floor).
—Dr. Vinny