THese are layers, yes, not meat chickens? In which case I would suggest at
least 6 sq ft per chicken (ish, and that's a minimum), which would mean something of a nearly-unmanageable size, like 8x12. It can be done if you have very flat ground and not much in the way of predators to be defended against; you might look at broiler tractor designs, as they are the only ones that are regularly that sort of size.
You *could* go smaller, but it is asking for trouble IMO unless you can move the tractor twice a day.
Will they still have a run, for when they are back at the coop, or will this be their only outside time?
(e.t.a. - I would not put a wire floor in. That negates much of the benefit of the tractor by preventing them from being able to scratch and making it extremely difficult for you to move the tractor easily!
If predators are a concern, all you can do is make it as sturdy as you can (while very-lightweight -- a difficult engineering compromise for large structures) and put an apron around it to discourage things digging in. Look at the tractor pages here and you will see some examples)
Good luck,
Pat
