It depends on the breed.
The heavy breeds can fly out of a 6 foot tall fence while young, but once they are full grown, they usually can't get their big fluffy butts that high in the air.
Our laying hen run has 6 foot tall fencing. We always have some that fly out while they are young. We clip one wing of those hens that fly out. It stops it on some and others still will fly out.
Then we just catch hens every day until they get too heavy to fly out.
We usually only have about 4 or 5 out of 150 that continue to fly out after 1 wing has been clipped.
My Ameraucana's can still fly when full grown. I had one hen that would fly over the fence, go into the big coop, lay her egg, and then fly back to her own run.
If you are planning on having a light breed, then you will have to have a top on your run to keep them from flying out.
If you are getting Silkies, they are not known to fly much at all, so a shorter fence will hold them.
Hope this helps you.
Jean