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Having had IBM 4381 serial # 121 (first customer install) I can say it was about double the size of a side by side refrigerator.
The box was only half full as Model Groups 1 and 2 were uniprocessors, and the space was reserved for the Model Group 3 dual processor .
It did replace a 1 MIPS IBM 370/158 that was eight times its size.

http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP4381.html

http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP3158.html