Answer: A 6d nail is 2 inches long. Note: The "d" means "penny." For example, a 10d nail is a 10-penny nail.
And how did "d" become the abbreviation for penny? The denarius was an old Roman coin and, as the centuries passed, the abbreviation "d" eventually became associated with the old British penny. The nail sizes go up ¼-inch (1.905 cm) for each penny. A 7-penny nail is 2¼ inches (5.715 cm) long.
This system originated in 15th-century England, when the "penny" size determined what one paid a blacksmith to forge 100 nails of that size of nail. You could buy 100 10-penny nails for 10 cents, or 100 16-penny nails for 16 cents, etc. This price became obsolete before 1500 AD, but is so entrenched in convention that its use persists to this day