A grown Great Dane is usually 110 to 175 lbs and measures roughly 44 to 50 inches from nose to the base of the tail. She is, by a distance, the hardest dog in this range to buy a bed for, most beds sold as 'extra large' will still have her hanging off the edge.
That matters more for a Dane than almost any other breed. When a dog this heavy sleeps half-on and half-off a bed, the joints that end up on the hard floor are carrying the most weight in the house.
Danes are commonly affected by hip dysplasia and osteoarthritis, and the breed is predisposed to wobbler syndrome, which affects the neck and spine. They also age fast, a Dane is a senior dog at around six or seven, far earlier than a Lab.
So: length first, then support. Egg-crate orthopedic foam spreads a very heavy dog's weight instead of letting it press through a few points.