How big is a city? Comparing the populations of incorporated cities is misleading, as Matt Yglesias notes, because you ignore the suburbs. Comparing the size of metropolitan statistical areas (cities plus suburbs where commuters live) makes a lot of intuitive sense. Seattle "feels" about the same size as Minneapolis, and it is (both have metro areas a little over 3 million). We could divide U.S. cities in this way:
Metropolis (> 10 million): New York, LA.
Big Cities (5-10 million): Chicago, Dallas
Major Metro Areas: (5 million): Philadelphia, Houston, Miami, Atlanta, DC
Medium Cities (4 million): Boston, San Francisco, Phoenix, others
Small Cities (2.5-3.5 million): e.g. San Diego, St. Louis, Baltimore, Denver
"Livable" Cities: (1-2.5 million): Kansas City, Providence, Richmond, etc.
Provincial Cities (0.5-1 million): Des Moines, Madison, Wichita
Any smaller than that and it's more like a college town.