Well, my round pen is 15 pannels that are 15ft long (big horse panels). I've been out of school for a while, i know i can convert that to an equation and find the width. But i think i will do it the easy way and just measure it today
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.
I don't have one built yet here at the new place, but at our old place we had two: one was a 40', and the other a 60'. The 60' one was tougher to work horses in since they were so much further away, but it made for a good workout for ME, lol! I'm thinking that I will make my next round pen about 45'.