You mean for the run, i.e. the outdoor fenced area where the chickens can hang out?
It is certainly true that for a given length of fencing, you enclose the most area by making it a circle (or circle-oid figure that incorporates the coop=henhouse as part of it).
HOWEVER it is much, much more difficult to build a really STRONG circular fence, so unless you are ok with it being fairly wibbly and unbraced, round is not an easy option. Ditto for octagonal. Not that they can't be done but to do them right is a PITA with a bunch of angle cuts to your horizontal fence lumber. Also it makes it harder to put a top on the run if you should ever wish to do that, and is a bit more wasteful of material if you do a wire apron for digproofing.
So, a square is usually the best compromise between perimeter-to-area ratio versus ease of construction.
Except that if you want to put a strongly-supported roof on, it may be better to do a rectangle because even though it makes less-economical use of your fencing material, it enables you to have a narrower span to the rafter-type thingies you are supporting your roof or wire top with.
Therefore I would say that the most correct answer would be "it depends what you're trying to do"
Good luck, have fun,
Pat