Dog height is measured at the withers --- the highest peak of the dog's shoulder blades. The American Kennel Club sets standard heights for dogs: for Great Danes the male must be at least 30 inches at the shoulders and preferably should be 32 inches or more; the female should be at least 28 inches, but it's preferred that her height be 30 inches or taller. In a dog show, Danes under minimum height are disqualified. A special tool called a wicket is used to measure dogs for the show ring, to get uniform and accurate measurements on all the dogs. Wickets cost a couple hundred dollars, though. If you are curious about the height of your Great Dane but don't require exact show measurements, you can easily measure the dog at home with a few simple tools.
Position the dog in on a level surface with one side against a blank wall. Have a helper hold the dog in position if it won't stand still in a stay position.
Place a carpenter's level across the dog's withers. Lay the level at the highest point on the dog's shoulders.
Rest the level across the dog so it's touching the wall. Adjust the level position until the bubble is directly in the center.
Mark the wall at the bottom of the level with a pencil.
Move the dog away and use a metal measuring tape to measure from the floor to the pencil mark to get the dog's height.