If a horse prefers big open areas I start them over cross country type fences out in the open, as that is where they are more comfortable & will learn more easily. If they are equally good in an inclosed area or better I will start them in there. At my barn we have no outside inclosed areas so there isnt a huge difference between riding cross country or over stadium fences.
If you want you can always put some jump standards up in a field or on the trail to practice over. That way you can have the stadium fences along with lots of open space.
Why is starting an arab different from starting any other breed ???