I am looking for statistics on total warehouse space (square footage) in the U.S.
The U.S. Energy Information Administration ( http://www.eia.doe.gov/emeu/cbecs/char99/intro.html ) periodically does a survey of all commercial buildings by type of building, square footage, and other energy-related characteriestics. To supplement these data, you might also want to contact a group called McGraw-Hill Construction Dodge that generates information on new commercial construction for value and square footage.
Data from both of these sources may be found in the Construction and Housing section of the online "Statistical Abstract." Additonally, the Transportation Annual Survey provides some data on warehousing for the years 1989 to 1998.