The land portion within the red line in this map of Chicago is "Chicago's San Francisco" at just over 49 square miles and just under a million people. Additionally, enough parts of this area are growing that it will probably cross a million people in 2020, giving it an average density of about 20,000 ppsm. Although there are a variety of ways to describe Chicago, the area in this is most of what some would call the urban core of "Global Chicago," i.e. the part of Chicago influenced by and influencing other global cities. This doesn't include the area around the University of Chicago mostly because it would be hard to have a contiguous connection between this map, nor does it include the several suburban clusters that contribute to Global Chicago as a concept.
You may note that most of this area is relatively well-served by expressways (especially if we count Lake Shore Drive as an expressway even though technically it's not), as well as rail transport (both the 'L's colored lines and Metra commuter rail's dull green lines).