The Aalsmeer Flower Auction, the largest flower auction and one of the largest buildings in the world by footprint, with a whopping 518,000 square metres (51.8 hectares).
I don't know if you guys have been, but the Grand Canyon doesn't even look real in person. I've never felt the actual size of something until I saw it last year........
Took this photo when I made a trip to Zion National Park back in April. Most of the pictures do no justice to the scale of the cliffs, but this one is the closest.
They really make you feel small.