My take on this is:
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There's a raft of contradictory information out there - initial sources are:
http://www.census.gov/population/www/documentation/twps0027/twps0027.html
http://en.wikipedia.org/wiki/Historical_urban_community_sizes
https://faculty.washington.edu/modelski/WcitiesH.htm
http://www.odi.org.uk/events/docs/1125.pdf
http://geography.about.com/library/weekly/aa011201a.htm
http://en.wikipedia.org/wiki/List_of_largest_cities_throughout_history
It would seem also that there may have been cities greater than 1 million earlier, but the lack of census makes these estimates.
Potentially populations over 1 million (which were then not over 1 million by 1900):
Alexandria - 100BC
Rome - 0AD to 300 AD
Chang'an 700AD to 800 AD
Baghdad 925AD to 1200AD
Kaifeng 1200AD
Hangzhou 1200AD
Nanjing 1400AD
p.s. a proper list here that I've had to make myself, rather than find one that someone else has done and repeat. Hooray!