So, there is this old tale about Carl Hagenbeck, whether it's true or not... Nobody will know. But it's even included in his biography and told on the signage next to his statue.
When Carl Hagenbeck had the idea for the world's first big cat enclosure without bars, he asked himself the exact same question. He had previously bottle raised a Lion cub and started to teach him tricks including jumping over a vertical wall and from one platform to a second one to determine the jumping abilities. Legend says that the Lion named Truest managed to jump 5m distance. But Carl Hagenbeck took a good look at him and decided that he was probably not the sportiest lion ever and therefore built a 7m wide moat. Which ist still successful as enclosure barrier after 114 years.
So, long story short. I would go with 7m. And the wall on the visitor side a little higher than the one on the lion's side.