For years I've shared this book with kids I've taught and they've loved it. Although it's a picture book, even Middle Schoolers and the occasional High School student helping with younger kids have found it interesting. I even learned a few things I didn't already know from it myself.
Okay, I won't keep you guessing. Here's the title: Is A Blue Whale The Biggest Thing There Is?
Robert E. Wells is both the author and illustrator and he does a wonderful job of communicating the size of the universe in an amusing way.
The book is both fun and informative. My copy was published in 1993 but the book is still in print. That's no surprise, because it's so good. I recommend it highly.