Ray at a year old sometimes used to go over to our neighbour (4 ft fence, either under or over) to go play with the GS or steal their toys :D. But that was just a phase and he stopped.
Sugar once got over a 7 ft wall. We were in a panhandle (completely surrounded by houses aside from a driveway out - which the dogs didnt have access to) and she was just gone. After going around the neighbourhood looking for her - even though there was no way for her to get out - we went back home to find her looking at us through the next door gate. The house was empty and we had to get two ladders - one either side of the wall and carry her back over. We could never figure out how she got there till hubby saw her clear the wall again - chasing a cat !! Those were the only two times in her 10 years that she jumped, never before and never since. Our walls now would be a breeze for her, 5 ft in places. I think she was more intent on getting to the cat than actually getting over walls.
There also used to be a boxer on our road who used to dig under their solid wall. We were forever taking him back home.
I think 6-8 ft would be fine as well, as long as she is happy in her space and kept busy I don't think she would want to go anywhere