The population of Metropolis is about six million people.
How do I know this? Jimmy Olsen, of course.
Reading the Superman Family Showcase (which, as everyone knows, is really the Jimmy Olsen Showcase but DC didn't have the guts to call it that) is opening my eyes. I am learning about new things. How to use 4th dimensional forceps. Sending morse code messages with chalk dust. How to unthaw a dinosaur. I can't wait to show off at my next high school reunion.
Anyway, the story "The Six Jimmy Olsens" taught me that six people named Jimmy Olsen live in Metropolis. Now, trust me, you do not have the strength to know anything more about the story than that. Even I, whose Silver Fu is strong, so very strong ... even I, after reading the story, am now sequestered in the same urban Buddhist monastery where Eel O'Brien recovered, where I will need a semester of silence and separation from society to regain my composure. But the Absorbascon must go on.

Let's assume Metropolis is not significantly different statistically from our own world. Okay, that's a pretty big assumption, given that Metropolis seems to have about 10 times as many reporters as it does police, and the reporters are the one who have to stop all the crime. On the other hand, I live in Washington, D.C., so that seems pretty normal to me.
So, given that there are 296 people currently in the U.S. population named Jimmy Olsen, that means for each person named Jimmy Olsen there are just over one million other people.
Ergo, if there are six Jimmy Olsens in Metropolis, it's population should be about 6 million people. It also means that in Central City, there's a Jimmy Olsen on every block.
*Shudder*!