The smallest for a round pen I think I would go is a 35 footer.
Personally, I believe if you have a smaller space, you have more control over the horse. Like a 35 foot pen for just lunging and then as you start riding get a bigger one. Like go from a 35 foot lunging one to a 50 foot riding round pen.
I ALWAYS would recommend you teaching the horse to lunge OUT side the round pen and on a lunge line before you put then in a round pen. If you pressure the horse to much, he'll go right through or try to go right through the panels. Which will end up in a injured horse either way. If you get them to understand the lunging on a line, pretty good assumption they'll understand it off the line in a round pen.