I used the mobile home axle. In the thread CYJ mentioned length of the arms. The longer the better.
My first attempt I had 15' arms for a 30' circle. That was way small. The dog could not open up and run as well because he was in a sharper turn. I added about 9' to each side (ran out of room) and it was much better. I wish I could have went out even further.
I miss using it. My land is on a slope and that was about the flattest spot I had. I ended up having to decide between the jenny and my shop. The shop won out.
Ive built a bunch and they are still going. My advice is make it as light as you can and at least 28' arm. And keep it put up wher no one can see it even from air!