The double corona is one of the large classic sizes. It is longer than a Churchill and usually a little thicker. Traditional size charts put it at about 7½ to 8½ inches long with a 49–52 ring gauge. The Cuban factory size, called the Prominente, measures 194 mm (about 7⅝ inches) by 49. New World companies use the name more loosely, and some sell a 7-inch cigar with a ring gauge in the 50s as a double corona. Go by the printed measurements.
The name does not mean twice the size of a corona. It is a straight-sided parejo with a rounded head and an open foot, and it takes a long time to smoke: well over an hour, often closer to two. See smoking time.
It is less common in shops selling New World cigars than it once was, because many brands have shifted toward the toro and the gordo.