You can never have too much land for them to roam, in my opinion. The more they have, the less likely they will strip out the grass and over fertilize the ground. Back when I let mine free-range, they stayed within about 200 feet of their coop MOST of the time. A few of them would go a little further. I have five acres and I think they used just about one in total simply because they stayed where they could easily see their coop. For only four or five hens, a half acre will be perfect. Not too big and not too small. Fencing in the third side is a good idea since it will lower your risk of predators. You'll still have some but not as many. Mostly, I'd be concerned about hawks and owls. Both are bold and will attack even if you are nearby. On days that you see them out and about, I'd be extra watchful.