Well this is what I have
50' round, because that's all the space we had, needed to fit between the cattle race and the shed there.
Bought the fence posts at a farm sale cost me $3 each, two utility poles, cut down for the gate posts, $5 each at the same sale.
The rails are 2nd grade 2" x 4" 8' long rails, cost $250 for a lift of them and I used just over half on the pen, and traded the rest against the gate and the sand.
Then it cost me a bunch of nice meals and other favors
to get DH to level the site and set the poles etc.
Works well enough for my guys who are all pretty laid back, not sure it would bounce a wild one off of the rail, keep meaning to put another rail, heavier one, along the top there.