Fencing in (and out) chickens... How High?

We're getting our first chickens (layers/maybe dual purpose), and are also planning to fence the one-acre yard. I'm fine with the hens roaming the yard, but how do I keep them in? We won't be getting banties, and I'm hoping to find an old breed that stays put more or less but also forages well. But my main question is: how high does our fence need to be, or should I just resign myself to a portable coop (ark/tractor)? I'm fencing the vegetable/flower garden as well.

Hope this question isn't excessively dumb: when you're starting out you never know if you're asking unanswerable things, or just completely obvious ones. Thanks...