How big was the first Computer?
Let us part from the premise that the first patented computer was the ENIAC (Electronic Numerical Integrator and Computer) , originally invented by J. Presper Eckert and John Mauchly at University of Pennsylvania in 1946. There were several other models created prior but this one was patented (the inventors later lost the patent) and fully functional by the time it was finished.
This stand-alone fixture weighed 110,231 pounds (50 tons metric), and basically took about 1,700-1,800 square feet. An average machine of this kind would have reached regular ceilings because the average foot-to-ceiling height of average buildings by the 1940's was around 7-8 feet tall, which is the height of the machine. It would have been environmentally hazardous to produce these machines in mass.