I do free lunging with my horse in a 30x60m (approx) ring, I use a lunge whip because I find it is a better more versitile tool. I just lunge him and usually he will circle me on his own without a barrier and sometimes he goes to the other end and gallops back. Even in a bigger ring, he still joins up and does a full follow up.
The only reason to use a round pen is it keeps the horse focused on the human, and they realize quicker that the human is the leader. I can't even do join up in the round pen anymore because the second we go in he sticks to me like glue and if I do manage to send him away, he signals me before he even gets to the rail.