I've tried to envision the smallest possible pit for Rio to test and it doesn't really work. Ya, I have heard 3+ km in at least 1 direction for the pit size. That's a whole lotta football fields.
Also, my understanding from other conversations is that the biggest issue for Rio is the pit slope and overburden stability. So the way that they could " test" that is to start the grand mine pit and see what works and what doesn't. But I am not at all familiar with the process of how they prove the parameters that went into the pit design other than a real life setting.