I don't have one, but I worked horses at one with a 30' diameter (wayyy too small) and the ranch I worked at had a 60' diameter, solid wood walls. Nice for young horses, but not for ornery one-eyed Appaloosas who would charge you
For most peoples' purposes, don't go smaller than 60' diameter -- that will let you work the unmounted horse at all gaits; ridden, you can work nearly all horses at the trot and reasonably balanced horses at the canter/lope, and have room to reverse at a trot or very balanced slow lope. If you have a large or unbalanced or wants-to-to-fast type horse, 80' (by which time it is more of a small ring than a roundpen, so it is easier to make with straight rather than curved sides) is honestly better to work in.
For just unmounted walk/trot work, or for use as a tiny turnout pen, or other less-usual purposes like that, down to 30' is okay -- but you can't do much else in that size.
i had a 40 foot store bought metal round pen, and the filly i was working with, the first thing she would do when turned loose to be worked, was lay down and roll in the sand. till one day she rolled too close to the panel and when she tried to get up somehow she got her hind leg cought betwen the bottom two bars, and almost broke her leg.
now i have a 40 foot homemade round pen. we made our pen with 5 inch posts (12 of them) every10 foot, and 10 foot long 2X6 at 2, 4, and 6 feet high. it is pritty solid, havent had anyone go threw it yet. we do ground work and start yong horses in it, and we ride new horses in it.
a friend down the road is a trainer and he has a square pen with 8-9 foot tall solid sides and is 50 foot square. he is a wonder with a mule or donkey!!
60' for mine. I wouldn't go any smaller or bigger for training. The 60' will keep you in control while ground working, but a safe distance that 30' won't allow. Once your more into the riding phase, I'd do more of a arena size 100' X whateverbyou can do.
Mine isn't round. It's a rectangle--55 ft x 65 ft. I originally created it for a large garden--pretty duh moment--then got my fence people to include it when we re-fenced in 2008. The more I use it, the more I like it. It's big enough to canter it, it makes a nice newhorse turnout, and I put them in there after a hard workout, to relax for an hour before they take a drink.
I tend to agree with 60 feet. I did 70 for the one here as I had the room and 2 things I have noticed since. One, I'm getting more exercise. This is not necessarily a bad thing however if your working a few colts in a day it's not good to have to move that much. A defiant moment in a horse and they can feel far enough away from you to make you move in unless your really good at throwing a rope toward them. The good side is I got better at directing a rope here. Two is that for the first time ever a horse has felt they had a good enough approach to try jumping out after cutting a diagonal. This is a wood fence and low compared to others I have built and worked in though so I'd have to say it a combination of the 2 measurements. It has changed the way I present some work to horses and that too is not a bad thing.
My previous were 50 and 60 feet and I'd have to say for all purpose 60 was my favorite. 50 is good for starting and all ground work but the horse often feels in close enough to you that they just do rather than make a choice so once out in a bigger area you might find them not as good at free work as you thought. Depends on your goals.