40'x 40' fenced in area....

More then plenty big enough you could fit 15+ with extra room in that size that is almost double what I have for 8

 I have 700 square feet and 40X40 is 1600 sq ft based on 10 feet per bird that is the min per bird so really you could fit over 50 in that size giving each one 30 feet per bird. you are giving them 200 feet per bird. But that is going to cost a petty penny on fencing

Coincidentally, 40 x 40 is the size of Premier's electric netting. I love the stuff, of course you will need an energizer.

I have fenced with t post and chicken wire, welded wire and this electric fence is the berries. 2 people can move it in less than 20 minutes and it keeps the predators... except flying ones out.

60X60 max on birds is 360 as 10 feet per bird and 60X60=3600 sq ft

more then 100 birds with allot of extra room that is allot of feed and bird poop

A lot depends on how quickly your climate allows grass to grow and recover.  Here, a half dozen, full sized hens scald off a 40x40 within a month, reducing it to bare dirt.  Just put that in your calculations and planning. FWIW.

If you have that kind of area for a roo and 8 hens I would divide it in half and then let them in one half for a week and then use the other half the next week so the ground isn't worked over so fast.  I move our 10x10 hoop house back and forth from 3 spots in the yard so they always have fresh grass.  I move it once or twice a week depending on how the ground looks.  The first spot is always green again by time I get it back to that spot.