Thanks, Chris. If we are going to use the area of concrete that is already in place in the garden, then that will determine the size of their basic run. It is roughly 9 metres square, so if I get 6 hens, that will be 1.5 sq metres each.
But as I said, the plan is to build the fruit cage, which will have a larger area, adjoining the run so we can let them out into that on days when we are around to shut them away again at night - which will be most of the time except on some short winter days when we are away until after dark.
The previous run we had was something over 18 metres square. We had 5 chickens at the least and 8 at the most. So with 8 they would have had more than 2 square metres each. They appeared to have plenty of room there and we never had serious fighting or pecking problems, just the usual bit of squabbling when new hens came into the flock. But I did worry about the ground getting stale as they were in that one space all the time.
Obviously both spaces will be tall enough for us to walk in, so they will be able to fly up to perches. I think vertical space as well as horizontal makes a difference.