How big is the Mission Control Center at NASA?

NASA actually has several Mission Control Centers, though the main MCC, and the one most people are familiar with, is the Manned Flight Center (MCC-H) at Johnson Space Center in Houston, which directs all Manned Flight activities (e.g., Shuttle, International Space Station, etc.) Up until the mission clock starts advancing, Kennedy Space Center Launch Control is in charge of the mission; after the countdown ends and the mission begins, control switches to MCC at JSC.

Since MCC-H is in a hurricane sensitive zone, there are backup MCC facilities at Kennedy Space Center at MCC in Moscow for ISS operations.

There are actually several Flight Control rooms in the MCC building - each is manned by a dozen up to 20 controllers.