We have a 6' privacy fence around our entire yard and for the most part it seems to do the trick but I have a clever hen that lets herself out of the yard and can't figure out how to get back in, yet she's smart enough to peck the front door for me to come get her and put her back in the backyard.
I've been very fortunate and haven't lost any to predators yet because they mostly come out at night, but we have very large birds of prey. A Red Tailed Hawk grabbed one of my young Marans and flew up into a tall tree before losing its grip. It survived but I watched the whole thing happen and I'm still shocked the little guy made it! Scratched, bruised, and shocked but alive! A local friend wasn't so lucky on the other hand and last month a coyote killed 17 of her chickens.