I have two acres, but about half is woods. Chickens will find a place to go to cover if you have trees and shrubs. My two acres is not even completely fenced, but chickens are noted for having bad eyesight and they never get very far from the coop where their water and feed is located. I think I read in one of the chicken keeping books that a chicken won't go much past 65 feet from "home." I started letting my eight week olds out for much of the day earlier this week and they start out eating grass near the coop, then they go into the woods to a little grove of young saplings and they rest there and occasionally go back out to forage and dig in the leaves for bugs and worms. They have never yet gone more than 50 feet from their coop. When it is getting close to roosting time, I call them and they come back to the coop to eat, drink and sleep. As long as you have some shrubs and trees for cover and as long as you lock them up at night, I think all of your chickens will be fine. The height of your fence probably won't matter, since the chickens can fly, but the fact that they won't go far from the coop should keep them in.