We have them. Excellent beds. You can hose or spray down the mattress and wash the covers which controls the pong of dog. The mattresses we have are almost 3 years old and show no signs of wear. One of the covers is a bit threadbare as one of the dogs likes to try to dig his bed up.
Size wise, the large flat mattresses would suit a great Dane. If in doubt, phone them and ask. Our mattresses are bigger than they need to be, and if they ever wear out, I'd go for the nests instead.
Mine both fit in the XL, they are 30kg and 25kg but the bigger dog loves to stretch out in it on his own. He likes to rest his head on the sides too and every other bed I’ve bought, the sides have collapsed after a few months but not this one.