I've never built a chicken tractor, but have lots of experience w/ things that roll. When building something for 'off-road' use, diameter is your friend. Small wheels will find every chuck hole and divot on your property, and drop into them. There are bicycle type wheels (up to 26") that are good for things such as this. They mount to a plain round axle. You see them on 2 wheel garden carts. You might even be able to scavenge a couple 20" bicycle wheels and rig them up somehow. I assume chicken tractors are very light, so you don't need anything heavy duty.

I used wheelbarrow wheels for my chicken tractor and I'm glad because they nicely roll over my lawn. They would be ok over rougher stuff too,if I woluld raise it higher. The axle is 5/8 and does flex when the weight is on it unfortuniately. My coop is 4 by 6 with the run that is 6 by 12. the coop is part of the run and wheels retract up and down and that is a pain to do and I need a better system. Make sure you build it as light as possible, it don't take long for the weight to add up. If you want I can send ya some pic's. If I had to do it over again I might build the coop separate from the run and have the coop on wheels inside the run. I've seen runs on the internet that are cattle fencing on a pullable wooden frame with a tarp over the run.

I'm sure there is probably a "right" answer to your question, but I'll just provide my best answer. I currently have 3 chicken tractors and the wheels are old lawn mower tires (little hard rubber jobs about 8" or so).

It cost me $2.00 for a carboard box of them, so I wasn't going to sweat the details.

Simplicity is your friend in this case. Larger tires would roll easier, but you are still restrcited by the gap at the bottom. A gap wide enough to roll over rough terrain will allow chickens to escape. Too small a gap will catch on every little hump. I keep mine about 2" at the back. Seems like a good compromise for my pasture. The chickens have never gotten out, but it still rolls OK.

For axles I just used lag bolts and several washers to allow the wheels to spin. You can only move as fast as the chickens walk, so don't get crazy with this end of things either. In fact I built my first tractor before I had the lawn mower wheels, and I used a 6" hole saw to cut 3/4 plywood discs. I planned to replace them with lawn mower tires, but they work so well I haven't done anything yet.

Good luck. Hope this helps.

Russ