Conversation so far:
What background can you give me on the Merchandise Mart?
The Merchandise Mart (or the Merch Mart, or the Mart) is a commercial building located in downtown Chicago, Illinois.
What other businesses participate in the Merchandise Mart?
It has become home to several other enterprises, including the Shops at the Mart, the Chicago campus of the Illinois Institute of Art,  and Motorola Mobility.

Question: How big is the building space?
Self-contained rewrite: How big is the Merchandise Mart building space?