"he cleared six feet" = He is taller than 6 feet. Good luck!

GuideDogSaint is on the right track. You "clear" an obstacle when you don't hit it. The amount of space in between is the "clearance." In a high jump, the athlete tries to jump over a pole at a certain height. You can hit the pole or clear it. If the pole is set at six feet and you clear it, your jump is at least six feet high. You don't know how much higher. So by extension "he cleared six feet" means "I am sure he was more than six feet tall, but I don't know how much more." This isn't a standard way to describe someone's height. You can call it a colorful turn of phrase.

I agree it's a bit strange. At first I thought it meant he had jumped over something six feet high!

Agree with Ian's answer: He is taller than six feet.