I'm starting a new project, and it's an 1870's era fantasy/magic/steam city for a Pathfinder campaign. This is the first time I'm doing a major metropolis size map that's not a traditional fantasy city. It would be a large, urban, port setting along the shore near rivers and mountains that's on the early side of industrial revolution, with a 5-6 story brick buildings, gas-lit avenues, railway, outer areas etc. and a population of about 1.5 million. Think late 18th century Paris, New York, London - Victorian.

What I'm having difficulty with is - determining the actual size of the city. How many square miles does it need to cover to support the population? What elements does it need to have for that time period/population? What should the scale/size of the map be?

Any thoughts or advice are appreciated. Thanks.