I first started out with the 10 x 12 tractor, but built it way to heavy= 2x 6 and 2 x4 lumber, full roof. After reading Salatin's book I see the advantage of a light structure.
So I cut the original tractor in half, so I have 2 6 x 10 footer and I built 2 more 8x8 tractors. Also built a brooder for outside w/ a removable floor, built in heat lamps, nipple waters and built in feed bin. The brooder is 4x 8 feet and is also sectioned off into 2 4x4 sections. So I get 100 st run chicks from the local hatchery. I keep them in the basement for 3 to 5 days in a lager rubbermade tote. They are feed 22 % fast grow from the local farm store. Then out side to the brooder they go ( keeps the wife happy), they get a 4x4 section to start , they go to a feed ration of 18% from the local mill. Depending on the out side temps and such i will pull the divider from day 10 to 15.
At about day 21 I move them to the 2 6 x 10 tractors, 50 in each. They stay on the same 18 % ration to slaughter. At week 5 to 6 I split them up again and bring the 2 8x8 tractor in the rotation. Now it gets a wee bit complicated. The cockerels stay in the 6x 10 tractors, hens in the 8x8. At 8 weeks the cockerels get butchered and the 6x10 tractors get a new batch of 21 day olds. The hens get a extra week or so then get butchered and the cycle repeats. I do a couple hundred bird this way and shoot to be done by end of July.
I found it is better for me to run the garden hose to the tractors w/ a stock tank valve (
tsc 10 bucks) and what ever pan or tub is around.
I dump them before I move the tractor and they are full when done with the move. Feeders are built into the side of the tractor , bin style, with a troughs from scrap spouting. I make them about 3 to 4 feet long and big enough to hold 2 five gallon buckets of feed.
If you go with 50 birds and a 10 x 12 tractor you will be at 2.4 sq ft per bird, that is out of the optimal range according to Salatin.
He says 1.4 to 1.8 sq feet per bird is best. I think he is on to something , but I will simply say the bigger the bird the more room it needs.
Good Luck.