Chickens don't stand at roost. They wrap their claws around the roost and sort of "lock" them in position, then hunker down and squat over their feet. It's actally a carefully worked out balancing act and is fascinating to watch.
The roost diameter is key to this. Too narrow and they can't get an effective grip, to wide and they dont grip at all.
Now, they can squat down anywhere, as evidenced by some of the crazy places they find to sleep away from the coop. But, their feet have adapted over millions of years to grip and hold them in low tree branches at night. If you are going to keep them in a coop to best advantage, it's best you provide a simulation of this - the roost as we know it.