Name the largest city in North America prior to 1790.

I’m betting that you named one of the biggest cities list on this result from the 1790 U.S. census, which indicates that New York only had 33,000 people and Boston only had 18,000 people.

You probably didn’t mention the settlement of Cahokia, Illinois, located just east of St. Louis, Missouri.   An article titled “America’s Lost City” in the December 25, 2011 issue of Science indicates that Cahokia was a “large urban complex,” perhaps home to as many as 50,000 people.  Cahokia was “in its heyday” in the 12th century, and it was “by far the largest concentration of people north of Mexico until the late 18th century.”  Although this excellent article in Science is available online only to subscribers, you can access this recent article on Cahokia by National Geographic.