QUESTION:
How big is the inside of the plane?

ANSWER from Del Lewis (Pilot, Flight Missions Branch) on May 19:
The KAO is a civilian version of the military C-141 Starlifter.  
Floor to ceiling distance is around 8-10 feet.  Width is in most places 
is 12 feet. The observatory uses about 90 -- 100 feet of the total length 
of the airplane.