Mine is 50ft in diameter and has solid sides 7 ft tall. I made it to work with half wild saddle horses, but it works great for minis. The solid walls helps the distractable ones from having too much to think about. It does take a bit more work on my part to start a mini in it lunging because I must walk, when I stand in the middle they figure out pretty quick I can't reach them and some of them get lazy.

However when I am ready to hitch them into a cart for the first time it is big enough to allow them to pull it comfortably.