I have Buff Orpingtons. they range around in the day and are cooped at nightand when we are not outside with them. Too many dogs and coyotes
They have a 12 x15 chainlink pen that is 6'. They can jump from the feeder and fly out till they are full grown- say till they are about 5-6 mos.
That being said with a good tail wind and some running room, they can fly over a 6' fence with no problem. Clipping one wing is all that is necessary, but you will have to clip fairly often.
I would start with young birds, coop them at first till they get used to the idea of staying around home and then give it a go. You could always get a very light weight plastic mesh fence to cover above your current fence- it would not require much in the way of posts, i would think some uv stable pvc or black pipe should work well.