Things to consider when deciding what size round pen to use.
Growing up in California, every boarding facility, show barn, and breeding farm had a round pen.
A standard lunge rope is 30′ long, and most people lunge with slack at the end, so traditional facilities and show barns generally have a 40′-50′ diameter round pen (generally called a lungeing pen).
When I began studying natural horsemanship in the 90’s, I found most of the professionals I was working with used a 50′ diameter round pen. This list includes Ray Hunt, Buck Brannaman, Leslie Desmond, and Pat Parelli. I would add though that in most cases, they were doing ground work and very little riding in the round pen.
John Lyons recommends a 60’ diameter pen.
I have personally played with over 200 different horses and I would also prefer a 60′ diameter pen. In addition to ground work (both on-line and at liberty), I like to start young horses and re-school problem horses in my round pen. I find if you intend on riding comfortably in your pen, you really need a 60′ diameter round pen.
A young horse under 2 years old can learn a lot in a 40’ diameter pen and could even be ridden at a walk. A wild horse or high spirited horse could be worked in a 50’ diameter pen, but if you intend on doing any riding faster then a walk, you would need a 60’ round pen.