The round pen here isn't actually completely round I left a corner in case of an emergency stop at the suggestion of my farrier (and a trainer).
Its grown from one that was roughly 45' to now 60' its made out of the portable corral panels such as the priefert ones which means as funding allows you can start however big you can afford then add more panels as you can.
Attached is the google earth image, the shadow of the tree covers part of the roundpen, but there is a gate into round pen then into arena, works nicely to do a little ground school in the round pen then into the arena for a good run. As you can see its not perfectly round but I have a round enough section I can keep one going if needed. (don't ask the cost of that many red pannels we don't like to think about it