I have a six foot, open top run. My first bunch of chickens (now over two years old) have never even thought of looking up and trying to fly out. However, my second bunch of chickens (now over a year old) were raised with Guineas... the Guineas taught my chickens how to fly over the fence.
Now - my chickens free range every day (almost all day long) so it doesn't matter for me. But, if I don't get out there soon enough to open the run door I have several that just fly out over the fence instead of waiting for me. So, even standard size chickens can fly over a six foot fence.