Free Range is Not Rocket Science
Choosing free range and determining how much room do you need for chickens is not an exact science. There is no set in stone formula. Most free range chickens do not receive the majority of their sustenance from eating afield. They require supplementation in the form of organically grown corn, feed from a feed store, or other suitable options year round. As such, you can just as easy keep a few dozen hens on nothing more than an acre of land as you could five or ten acres. The only difference rests in how much you have to supplement their natural diet.
Domesticated birds such as chickens, turkeys, grouse, and similar species, while retaining much of their natural instincts, are not breed for roughing it on bugs, beetles, and wild vegetation alone. These creatures have long been bred to be dependent on man for at least some of their sustenance. As such, even free range chickens need feed and roughage supplements to produce good eggs and meat to sustain a family.
Naturally, the more you challenge a flock to work to find food by keeping them afield without feed, the less you will have to supplement. The larger the area they have to roam and scratch, the larger their natural diet will be. However, at some point, egg production, overall flock health, and meat production will suffer without proper nutrition. Not to mention, most free range chicken producers will tell you that chickens seldom wander more than a few hundred yards from home, so a huge area is not necessary for a small flock.