This may make you scratch your head.
Our visible universe has a radius of about 46 billion light years, even though it's only 13.7 billion years old. There. It's head-scratching time.
The reason for this is that we aren't seeing the most distant points as they are now, but as they were when they sent their photons our way, shortly after the universe began. When that happened, the objects doing the emitting were 42 million (with an "M") light years away from where the Earth is now.
The reason it took that light so long to get here even though it was travelling at the speed of light is because as it headed our way it had to overcome space that was, and is, expanding at a VERY high rate of speed. Its trip here is analogous to a bowling ball rolling the wrong way on a very, very fast treadmill. Complicating matters still more, the expansion was slowing down for the first 9 billion years of the universe's life, but thanks to dark energy it is now speeding up. Over the life of the universe, the distance to the outermost objects we can see today has changed from 42 million light years to 46 BILLION light years. Whoops. You'll probably need to do more head-scratching.
Finally, those furthest objects that we can only just see today and which were very young at the time and are now 46 billion light years away have probably evolved into galaxies and stars much like those we have near to us now. That's because the universe was a big, homogenous lump of heat back then, all of it being much the same and probably evolving the same way throughout. Some of the stars that are now in those most-distant places are probably moving away from us. Thanks to the concurrent expansion of space, some of those stars are actually moving out of visible range even though they were in the visible range until now. Their light will NEVER AGAIN travel to us. Due to that, the visible universe is effectively losing some 20,000 stars per second. (Good thing it gains stars from other phenomenon, eh?)
That's how big the visible universe is. But how big is the ENTIRE universe? It's impossible to know.
It's kind of fun to think about.
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