The information from the passage that supports the idea that the Space Transportation System is very large is this: D. It has to be put together in one of the largest buildings in the world.
What is the supporting statement?
We reach this conclusion because putting together the STS in one of the largest buildings in the world explicitly indicates its significant size.
This building needs to be large enough to accommodate the entire system, including the Space Shuttle Orbiter, the Space Shuttle Solid Rocket Boosters, and the External Tank, all of which are massive components.