Quick Question on the begining of the Universe.

Ok. I'm watching Comos (great show). But something came up on the show that I have heard before that always kind of bugged me. The host (Neil Tyson Degrasse) holds up a marble and proclaims " the entire universe was once the size of a marble"..... and I'm assuming he means, pre-big bang the entire universe was just sitting there... the size of a marble.... and like I said earlier, this is not the 1st time I heard this. My question is this. Why a marble? How do we know the universe wasn't the size of a basketball? Or a small barn?..... Or a pin head?.....Or the moon? Why are we so confident it was the size of a marble?



That is all.