Yard is of little importance with the breed as it's not a sporting dog. People think giant dog = giant yard. But they do quite well in smaller yards as they're not high strung. First 6 months you're not going to want to run them hard anyways or at least I didn't due to bone growth being more fragile. Don't get me wrong they will want to play even when larger but they get tired pretty easily, and what you lack in yard you can make up for in walks around the neighborhood.
A high fenced yard isn't a bad idea if you have one to keep him/her from running around. But first year or so with mine I simply walked her when she went outside as I didn't have a fenced yard. Cropping(if chosen) of ears must be done early 7-8 weeks, and it's hard to find a vet that will even do croppings anymore in this day. And cropping in itself is alot of commitment and care for several months after it's done.
Also get ready for helping lift 140lbs plus in/out of the car, large food bills(nutro is good food and available almost everywhere) and possibility that a large number of them suffer from bone cancer. Also elevated food/water bowls are best(probably have to build a stand for dishes). As I said this dog will want to be right beside you. Meaning tv time it's gonna want the couch, bed time it's coming in the bed or making a damn good attempt. And at dinner time it wants what you're having.. Almost like it's bred into them..

I would encourage anyone considering getting one to be comfortable with a breed that
needs that much attention.
This isn't nearly all that encomposes the breed. Just some larger points to consider. All in all I hope that after research and should you decide to get a dane that you take good care of it. Nothing like them. And sad to see so many in shelters. It's an expensive breed to buy and maintain. Again pm me if you have any questions.
www.puppyfind.com list several breeders and you can search by state
www.petfinder.com has rescues if you'd like to rescue a dane. Many good dogs in rescues. Wish I had more room.