The tractor I have in mind would be a 4 X 6 coop elevated over a 4 X 12 run. The run is a wee bit small for them. But, being portable, I'll be able to give them fresh pickings often.
make it 4 x 8 and you should be safe with minimal extra cost.. I have the same for my 4 Silkies keeping them from our 3 outside cats but they have a special room in my new coop 1 of 3 not including the tractor it is 4 x 12 x 5