If you're asking about the size of the tractor itself, I wouldn't go less than 3sq ft per bird. 4 or 5 would be better. As long as they're getting new grassy area to forage through every couple of days, they shouldn't go after each other. If you're asking about how much area the tractor needs to cover before going back over the same area again (how much time for the area to recover), it kind of depends on a couple of things. It depends on how fast the grass (or whatever the ground cover is) is growing and how much rain (or watering) you get to dissolve the droppings down into the soil. Obviously, when the birds are smaller, they won't be leaving as much fertilizer. So you can either wait longer between moves, or cover a smaller area when they're little. When they're bigger, they may need to be moved more often (daily?) For a "normal" grass lawn with "normal" rainfall or watering to keep the grass healthy, I would think that 2 weeks would be plenty of recovery time for a day or two of "chicken trauma". LOL. Therefore you would need to be able to move the tractor 14 times before getting back to the same spot. But in the end, your best bet is just to try it and see what happens. Watch how quickly the grass recovers and how quickly the fertilizer is absorbed into the ground. It'll likely change with the seasons as well as the size of the birds. Good luck!