I have just over 6 foot high fences and even when I fostered an escapologist, no one could get over it.
Depends on the dog, type of fencing etc. Some dogs like to climb My Labs would be fine with a pretty short fence; it doesn't cross their minds to try to get over it. Others are more determined escapologists!
My Lab tore holes in a fence recently so I now have six foot fences lined with aviary fencing. Welcome to fort Knox!
I've gone for the Mott and Bailey approach. We have a six foot fence then another one also six foot outside that. They can't get out but more importantly no one can get in
My fence is only 5 foot high larch lap and none of the dogs I’ve had here over the years (springer, cocker, Lab and LabX) have escaped. When we first had our Springer, many years ago, we lived elsewhere with a 6 foot fence -but it was ranch rails and the dog managed to scramble over it. So I’d say the type of fence matters too.
Most of my garden is fenced about 5ft but there is a length which is only wire and shrubs about 3ft high and I taught them not to jump over and they haven't (up to this moment!).
We have 42 inch chain link for quite a bit of the back edge of our property. In the front it is the classic white picket (abs plastic) that is about 36 inches. Works for us. Makes the neighbors happy, they come over and pet Murphy when he puts his paws on top. Murphy does not seem to have any interest in escaping, but years ago we had a German Short Hair Pointer that could have starred in the Great Escape. Walked right up an eight foot chain link, burrowed under even after we had poured a concrete footing 12 inches deep. A real artist at his trade.
My Malamute was an escape artist, who would go over, under or thru most any wood fence and even chewed thru a wire fence once. Our Labs are just the opposite. We have a fenced yard with about 5' of fence and no problems with escaping dogs. We have a 8x32' covered front porch, and only a 30" high pet play pen fence across the 7' wide opening. Both dogs stay on the porch with no attempt to escape. Cooper could clear it like a deer. Cooper is funny to watch on the front porch, she will put her front feet on top of the railing when she is watching a person or other animal. She sometimes moves down the porch standing on her back legs with her front feet on the railing, but when she gets to the gate she stands on her back legs and does not touch the gate. When she stands straight up on her back legs, her nose is higher than mine, and I'm a little under 5'10"
I was visiting my inlaws in NW Missouri a couple of years ago and took our two for a walk at night. We went by a house that had a 42" chain link fence around the back yard, and a couple of dogs, including a large Black Lab. He barked at us and cleared the fence to see us. Fortunately he was friendly and just wanted to meet some new Labs. He actually stayed near his house after getting out of the fence.
