For what it is worth, I don't think that horse is as big as you were told. It also sounds like the person who owns the horse has not been forthright with you in telling you about the horses history. If she owns the horse, she should know who has ridden it and how it acted or responded as a problem/issue would go back to the owner quickly for resolution. Lungeing the horse is just going to build up it's stamina, not solve a problem. There are better ways to fix this with an investment of time, if you want to.