How high should my garden fence be to keep chickens in.?

Your best choice is putting a roof over the area where the chickens will be staying. That way you won't have to worry about the hight of the fence. It will also keep flying predators as well as ground predators out of the coop. and you wont have to clip their wings which would have to be done at least once a year. have fun with your new buddies!! :)