What and where are the largest vacuum chambers in the US (for use in
scientific testing)?

http://dmoz.org/Science/Instruments_and_Supplies/Laboratory_Equipment/Vacuum/

might help....

Boeing Satellite Systems in El Segundo, California has a very large
thermal vacuum chamber.

Boeing Satellite Systems 
World's Largest Commercial Satellite Factory
http://www.boeing.com/defense-space/space/bss/factsheets/isf/isf.html
"In 1998 the Space Simulation Laboratory was also expanded to include
a new thermal vacuum chamber. Weighing one million pounds, this
mailbox-shaped chamber is the largest of its kind and can
simultaneously test two Boeing 702 satellites. The working test space
inside the chamber is 63,000 cubic feet and measures 35 feet wide, 45
feet deep, and 40 feet high at the center. The chamber can attain
temperatures ranging from +250°F to -300°F. Additional space behind
the shroud allows maintenance and access to the hundreds of wire
bundles that relay spacecraft test information through the chamber
wall via hermetically sealed feed-throughs."


Googlenut