My most functional tractor was 3' x 6'. I got a lot of mileage out of that: Built it originally as a brooder. Set up in basement or garage with cardboard and vinyl under it, and deep litter. Then, moved it and the littles out into the yard, and banked it with hay bales while I finished building their coop. Continued to use it as a "day" tractor. Then, used it as a broody "day" pen. It took me less than a week to train the littles and Mama to return to the big coop at night. A bit more difficult getting them into the tractor in the morning. It was built of 2 x 2, w/1/2" hdw. cloth. Frame is now rotten. I intend to dismantle it, and give it a rebuild. I then built an 8 x 8, panelized affair with chicken wire. It will contain birds, but not at all reliable to protect them from any passing predator. .
