Our outdoor (and only) arena isn't all that large, but it serves it's purpose. We mainly use it for when the field is too wet or for beginners.
Once you can w/t/c off the lunge safely you get to ride in the field. It's about the size of two dressage arenas stuck together, and the main riding area is mowed pretty regularly. The horses are so much happier out in the open.
We also have some lovely trails, but the students really only go on trail rides in the summer.