The size most commonly used in the orchestra is called the contrabass-tuba, or double-bass tuba.

NPR: Young Tuba Player Gets Nod from Phila. Orchestra

In the "Ring, " four Wagner tubas join an enlarged brass section that includes other orchestral rarities like the bass trumpet, contrabass trombone and contrabass tuba, creating "choirs" of gleaming brass.

WSJ: It Takes Brass to Play the Wagner Tuba | By Barbara Jepson | The Ring of the Nibelung