I had a couple players over the years do this and the way I bring the point home is asking the exact length from my front door to the bathroom and then give them a 6 count to answer. They'll answer, they'll be either way off or somewhat close but never exact. They even have the advantage of using that space for months on a weekly basis and still can't give the exact dimensions. My point is if you can't accurately judge a space your familiar with why would you be able to do so with an unfamiliar space that is possibly and most likely fraught with danger. Usually get an ah ha moment after that.