My post are 10 - 12 ft apart and the pipe pens are 30 ft long and 18 ft wide.
The can hop and bounce around, roll, run up and down some.
When it rains its muddy. Only way to get them out of the mud would be a Barn with Box stalls.
1 1/2 acres of irrigated pasture with hay is fine for two horses. Depending on your area and what type of grass . I have two horses out on 2 and 1/2 acres and they get small flakes of hay and are fat. My pasture is irrigated and is the grazing bermuda pasto rico . This is also the type of bermuda that is used for haying. It is not just seed from the big box stores. I have 3 other horses out on a smaller pasture, and they are all fat (smaller horses that get along )