This is easy to conceptualize. If the universe started from a singularity and expanded in all directions, at the speed of light 13 billion years in one direction and 13 billion years in the other direction, that is at least 26 billion light years across. But space is expanding faster than the speed of light, not to mention ALL space is expanding everywhere. It's easy to see why the observable universe is 40+ billion light years across.

A better question would be, do we have any idea how large the entire universe is, including what we can't see? We must be able to calculate how much of the universe we can't see.