How big is Warehouse 13 (i.e. in square feet), and is it practical to build a warehouse of its size?
No official info to the best of my knowledge, but several places on the net mention that the zip line is ~2 miles long (~10k ft).
I don't recall having perpendicular zip lines, so the width of the warehouse is questionable - let's assume 200-500 feet wide so you can travel quickly to/from zip line.
That would net you 2 – 5 million square feet.
Is it practical? Sure. Boeing plant is 398,000 m² - 4.3 million sq ft (from http://en.wikipedia.org/wiki/List_of_largest_buildings_in_the_world ).
I just read an interview which has the following quote:
When the panel was asked how big the Warehouse actually is, the answers ran the gamut from extremely large to impossibly huge. Joanne Kelly said it was infinite, while Allison Scagliotti suggested it was constantly expanding, much like the universe itself. Saul Rubinek tried for a more prosaic answer, guessing that, since the Warehouse is the sum of all twelve that came before it, it must extend about three miles into the mountain. Jack Kenny suggested the precise dimensions didn't matter - what was really important is that we never see the back of the warehouse, that it continue to go on forever.
So, I guess it is quite large, and that it doesn't seem to have a back to it.