Perhaps the average length would be much higher if they werent all so damned proficient at committing suicide.
My rope, Fibonacci, was almost 13" when he decided to jump. The tank was as secure as any out there. He actually survived the first (and subsequent) jump. Poor kid-o was barely moving at all when I replaced him. I lowered the water level to 60% and put a net over the small opening in front of the filter. The next morning he was on the floor again. That time, I lowered the water level still further and treated the tank. He seemed very out of it at that point.
The third jump did it. My cats were all gathered around him when I woke up. They love watching the fish, him and the polys in particular--I was shocked that they didnt want to touch/play with/eat him. He was completely dried out after round three. Would love to own another, but me and the girlfriend were quite upset when we lost him :/