I have a lot of experience in this area. Live in rural Nevada and have 2 German Shepherds and 1 Husky (75 lbs, 70 lbs, 55 lbs.). Some would call these big dogs, but others might think of them as "medium sized".
For these dogs, fences are just a suggestion. They have gotten over any sort of fence up to 6 ft. (smooth wood slat, chain link, welded wire "field fence", even a wrought iron vertical post "no climb" fence)... If there is enough room to do a run up, they can potentially clear like high jumper. Otherwise it is a process of jumping to pull themselves up (like a chin-up), and then scramble over top . They will seek out the weakest point or any sort of horizontal crossbar or any available intermediate point for leverage / toe-hold. Gates and latches always seems to provide something like this.
I've also had German Shepherds in the past who had no interest in scaling fences, even low ones... Could physically do it, just wasn't their thing.
If you have an escape artist dog, then you will have to think about dog-proofing the fence. Here are some things to keep in mind:
1. Not sure where you are located, but most places in the U.S. will have fence height restrictions (as local government "ordinances") ONLY at the property line. Elsewhere on your land, you are free to do what you want.... unless you live in some sort of "planned community" which has its own set of restrictions (i.e. CC&R's).
2. Fence height is not the be-all, end-all. You have to remove or obstruct the escape / fence-scaling means. So, if the dog needs a run up to jump over/onto a fence, interrupt the run up path with hedges near the fence. If the dog is gaining leverage on a horizontal support, or latch, or... then move that item to the other side of the fence, or leave on same side and face in a smooth material.
Some people even dig trenches on the "inside" side of a fence, thereby keeping with the overall fence height restriction (e.g. 6 ft. from grade when viewed from the outside), but making it essentially a 7 ft. or 8 ft. fence to get over from the inside.
3. If you have a real jumper / escape artist, then you will probably have to go with either "lean ins" (think about the 45 degree angled fence 'topper' you might see around a prison yard, but WITHOUT the RAZOR WIRE!!!) or "coyote rollers".
4. The "invisible fence" (signal wire buried along perimeter of property, which activates a shock collar worn by the dog when he gets close) works in certain cases. Best as a training aid to deter a young dog from ever getting interested in the fence in the first place. Some dogs get shocked once and will forever stay away. With other dogs you have to keep the collar charged and on them at all times, which is a pain -- and after a couple escape-free months you will stop doing it (and a smart dog will KNOW that you stopped doing it... Ask me how I know).
5. As others have said, once you completely dog-proof the "over the top" escape method, a determined dog will dig underneath. Then you get into the whole discussion of burying chicken wire or chain link, or even having a concrete footing at the base of the fence.
