Moving a horse in a circle stresses a horses legs almost regardless of the size of the cirlce. Make it as large as you can afford to. I have a round pen that is 50' and it is a little too small to canter ahorse unless the horse can collect pretty well. I would make it as large as I had room for.
There's nothing like the Rockies in the springtime... Nothing like the freedom in the air... And there ain't nothing better than draggin calves to the fire and there's nothing like the smell of burning hair. -Brenn Hill