I agree with Aart. electrify that tractor. Plan on a minimum of 2 s.f./bird. 4 s.f. would be better. Will you let them out to range at all??? Other option would be to build a more stationary coop, which could allow increased s.f. and use deep litter. It boils down to this: Given the size of the coop you'll need, will you realistically be committed to moving it once, if not twice/day, rain or shine? I have 2 tractors, and one 8 x 12 coop that was supposed to be a tractor, but got parked b/c it was too heavy to move. All get used, but the 8 x 12 hoop coop gets used the most for brooding youngsters, grow out for meaties, rooster jail. Small tractor (1/2" hdw cloth) most recently used for broody/chick pen due to heavy hawk predation. That one (3 x 6) was invaluable in basement and later in garage for brooding chicks the first 2 years. The bigger tractor (6 x 8' w/1" c.w.) does not see as much use and is currently parked in garden for a cold frame. No matter what you build, it's helpful to build it with an eye towards alternate and future use. If you'll be brooding meaties in future years, you might look into electro net. It will give you 1600 s.f. and will be predator proof except for weasels and overhead. Great b/c it can be moved around the yard with minimal fuss, and give them LOTS of range opportunities.
