Danes typically don't need a whole lot of room in a house (heck we have 5 right now and live in 771 square feet)...just a lot of room on a couch and bed LOL. A lot of people say that they are the best apartment dogs, because they are couch potatoes.
But, they do need exercise to stay fit and healthy. I take my dogs out running a few times a week. Not just for a walk but several hours of hiking/running/swimming to keep them a peak performance. Just having a backyard isn't enough because most dogs don't get adequate exercise from just being put out back. They need to exert physical energy for about a half hour daily, or several hours weekly, to stay fit. Just like with people.