Powell's City of Books claims to be the "largest used and new bookstore in the world, occupying an entire city block and housing approximately one million books." I had the good fortune of visiting Powell's while passing through Portland, and it was by far the most impressively gigantic bookstore I've ever been in!
It's true that the store occupies an entire block. That's an awful lot of real estate. Not only that, but it's at least three floors tall. I say "at least" because there were lower levels where I wasn't sure if they were part of the first floor or a basement level. The books were organized by color-themed rooms, but in the short time that I was there, I couldn't figure out which books resided in which rooms. I had to follow the signs, and even then, I got lost a few times. How amazing is it to actually get lost in a bookstore?
One would expect that with a bookstore so huge, the selection would be just as vast, and it was. There were old books and new releases, used books and new, in every genre you can think of. I was able to find a couple of books that I'd been looking for that I haven't been able to find at other book retailers. (Yes, they're available on Amazon, but the prices at Powell's was actually cheaper.)
I wish I had more time to browse the store. I could easily spend a day there and not see everything. They even have a rare books room, which could have occupied an hour all by itself.
If you ever get a chance to visit Portland, or if you're lucky enough to live there, I highly recommend visiting Powell's City of Books!