Round pen size question?

A 50' diameter round pen is approximately 15 meters in diameter. That should be sufficient for ponies and many horses at the trot. A large horse may have difficulty if he is not fit and supple -- but you said your horses aren't that large, so I think you'd be fine. It sounds as though all your horses are quite young. If they're under 4, you should work in short increments anyway -- maybe 15 minutes or so -- to accommodate their short attention spans. As long as you keep your schooling sessions short, you will be fine.