I have a couple of driving ponies that are about 10hh. I don't have a round pen but I do have room for a 30' pen. I just want to lunge them in it. I know bigger is better, but I don't have room. Is a 30' pen OK for lunging these 2?
If you only have room for a 30' pen, then that is all you can do.
Larger is always better as it will take the torque from the body, legs and joints that occurs when a animal is made to do any "work" with their body in a arched position.
Lunging done correctly can give exercise and work a issue but it must be done carefully and not overdone.
What you do tracking left you must also do tracking right so equal use and building of the body take place.
Footing is critical in a round pen because you get a track, a rut so quickly many not realize the added stress to the body the animal endure.
Best of luck with your decision.