Five feet might be the minimum standard set, but if this yearling has not been handled and has not been alone, a five foot pen will never hold it. My round pen is 5' 6" and several horses have cleared it and even more than that have climbed out in the last several years. I've even had weanlings climb out if they did not have a friend. I used to let other people bring un-handled horses over, but any more, I won't let them bring only one over.
If I were you, I would find someone with a very substantial pen or stall and make a deal with them to keep it until you can get set up properly.
A friend that lives about 1/2 mile from me got a mustang last year at Pauls Valley for the mustang makeover in fort Worth. It ran head-first into a post the third day at his house. The pen was 8 feet tall. He was not even there, but when he came in that morning, it was laying dead where it hit the post.
He called the people from the mustang makeover and they said his was the 3rd one to kill itself and it was only 3 days into it. They offered him another one, but he figured it was not meant to be.