Young chickens seem better able to fly than older (heavier) ones. However, some are just more apt to fly up and out. My spouse had to retrieve one chicken from the top of our barn, but we think it had gotten on top of the a railing, then the open coop door (human sized), then onto coop roof, then onto barn roof, but we didn't actually see the chicken get up there - we just needed to retrieve it.
It seems to me that a chicken will be able to fly up and out over some fairly tall items given enough length to fly that high, or something to get up on to shorten the distance. We have a run that is 10' wide. They are able to easily fly up and perch on the 4 ft fence that is on one side of the run, and they can do this from fairly close to the fence itself. So, 6 ft wouldn't give a regular to small size chicken much trouble at all. We have 2 very big Black Jersey Giants - they don't attempt to fly over anything. But, the lighter chickens sometimes need their wings clipped.