“If the doors of perception were cleansed every thing would appear to man as it is, Infinite. For man has closed himself up, till he sees all things thro’ narrow chinks of his cavern.” -William Blake
When it comes to the ultimate question of the size of the Universe, we have to look to greater scales than what we can possibly observe. Although we can place constraints on how big the unobservable Universe must be, coming up with a lower limit to its overall size, there’s a bigger question that we don’t yet know the answer to: is it finite in size, or is it truly infinite?
Beyond what we can see, there ought to be more Universe just like our own, originating from either the same Big Bang, or possibly, if inflation is correct, from other Big Bangs at later or earlier times. And thanks to the ideas of eternal inflation, we have very, very large numbers for what’s possible as far as size goes. But there’s a long way from very large to infinite, and determining whether that’s true is a very difficult prospect.