When I told her about my novel idea, my agent said, "Think big." My editor has said this to me before, as well, during a revision of my last book.
The first time I heard this term, I thought of it very literally. My 14-year-old daughter is in the midst of a "big book" extravaganza. She read Gone With the Wind, 
just finished The Thorn Birds, and is turning next to The Winds of War -- all books that are well over 1,000 pages and whose stories are sweeping, war-torn and epic. It only took me a moment to realize that this definition of "big book" was irrelevant to me. No one would ever describe what I do as sweeping or epic.
"Big book" could certainly mean "commercially successful." An agent and editor could well suggest that an author write a commercial book -- and "big book" is a term people use when a book hits the jackpot -- i.e. Eat, Pray, Love was one of the biggest books of the year. But again, I believe this does not apply to what I do. I wrote about this in my first post in this series on writing a novel.
Once I got those definitions out of the way, the book that came into my mind as an example of a "big book" was Elizabeth Strout's Olive Kitteridge. 
I read this book before Oprah and before the Pulitzer Prize, because I am a fan of Strout's earlier work. It is, on the one hand, the smallest book imaginable -- a relatively thin book of short stories about an unpleasant, aging woman in Maine. But you know the moment you start it that this book is "big." Strout holds nothing back. She gives us a whole world through this one woman and the people who know her. The writing is rich and deep and generous. You feel, when you read about Olive, that Elizabeth Strout has given something of her soul, and the experience takes your breath away. At least it did mine.
So that is what I believe a big book to be. Big in heart. Big in spirit. The book I am working on will be my seventh. Did I give my heart and soul to those other books? Of course I did. But the heart and the soul must be like any other muscle: they must get stronger as you use them. So, this time, I will try to give even more. I will try to expand my capacity for seeing my characters and my story. I will try to... think big.