I have a 6' fence with large fowl. The truth is if you have the occasional bird who just insists on getting out, she will get out. I have watched them "walk" up the fince, flapping and hanging on, til it got to the top and over, several times. Of course, she was tring to get back in within the day, but.....
I also have a large yard with no top. No, it's not predator proof... but has not been breached in the 3 or so years it's been up. We have hawks, but I think in this area they are small enough, or maybe just well fed enough, that they don't try to take a chicken. Oddly, we had a buzzard get in there reentl, apparently for the feed. No problems other than that.
It just depends on your area, and maybe on how hungry your wildlife is. If we had bears, this setup would not work. We also have a couple of dogs running around the property, often hanging out atround the chicken yard. I imagine they discourage any fox or coyote that is looking for a meal..
There are different kinds of wire fencing that you can use. If you want more securit, put a 1' or 2' sstrip along the bottom, so weasels, etc. can't slip through, and coons can't tear through. Mine has holes large enough that a coon could reach through and grab a chicken, but the chickens don't hang out next to the fence, they hang around the bushes that brow here and there in their ard, or in the coop on hot days. (It is about a 70' x 70' yard, with 9 birds.) My fence actually looks like chicken wire, but it is very heavy gauge, similar to hardware cloth.
Standard shicken wire will not keep anything out or in but chickens. Even a small dog can easily tear through it. It has its uses, as to fence the garden off from the chickens, or to partition a small area inside the coop, but offers no protection.