I've noticed that most are pretty small. I will have around 30-40 heavy breed chickens and would like to build an 'A' frame tractor (or 2 ... or 3) big enough to keep at least 15-20 birds in pretty much all summer. They will have the opportunity to roam every afternoon but must be protected at night and while I'm at work. I live in predator central.... Is there a reason most tractors are on the small side? Ease of movement, structural considerations, space limitations etc. I was thinking a 12' wide by 12' long tractor would be large enough to keep them happy. 12x12 or 144 square feet is 7-10 square feet per bird. It'll have a coop at one end 2 ' off the ground for shade and to give them as much outside area a possible. It will be about 9'x6', 54 square feet or 2.7-3.6 square feet per bird. Not ideal but doable. I have 10 isolated acres so it's not a space or neighbor issue. And I have a tractor i can move it with if it's too heavy to move by hand.