I've done the work of ranking the cities by size. White's approach to determining the biggest cities seem to down to two factors;
1) Spread the wealth. Because of America's more diverse geography, and fast build-up in a shorter amount of time, we don't see an monopoly of urbanization like with the Mediterranean, which had history, climate, and access to routes on its side. There's more of a give and take with the us--warm but swampy, fertile but cold, central but landlocked, ideal but isolated. He generally tries give one or two major cities per region--northeast, desert, etc. However the Feudal Core gets two super cities due to its size, and the Great Plains don't get any.
2) White also seems to lay things at the Rule of First Arrivers. If you divide things by the northeast, heartland, gulf coast, desert, and pacific northwest, the largest cities are the ones which were able to first exceed a population of 25,000. (Buffalo is also the first city on the Great Lakes to do so, and one that's still somewhat Northeastern) This makes sense, as these would be the cities that were able to still thrive when "roughing it", and without depending on modern things like highways, or even trains. The major aberration would be Sacramento in California, (California being a geographic entity unto itself) which was not the first city to get that big--San Francisco was. This exception may be due to the nature of Imperial Capitals in olden days.
Tier One: New Orleans, Cincinnati, Portland
The "Big Three". New Orleans, Cincinatti, and Portland would make up a big three, with Sacramento smaller than the other three, but bigger than any other city. Cincinnati would be the largest city in the midwest. It's status as a "northern" city is up for debate, it seems to be directly where north and south in America meet. It is the largest city in the Non-Denominational world. Interestingly, the first tier of cities is the only one that doesn't have a Non-Denominational majority. New Orleans is the largest city in the south, and the largest coastal city. Portland is largest city in the west.
Tier Two: Sacramento, Philadelphia, Buffalo, Salt Lake City, Louisville
Sacramento is the largest city in California, and is actually the only city in its size range. It's larger than any other city but "the big three" (and could be put in the first tier as "The Big Four". Philadelphia and Buffalo are the largest "northern" cities. Philadelphia is the largest city to be connected to the Atlantic Seaboard (although it's still somewhat inland), and the largest city east of the Appalachian mountains. It's also the only top ten city of industrial America to comparable in rank to Medieval America. Buffalo is the largest city of the Great Lakes, and there is no city north of it that exceeds it in size. Salt Lake City is the largest city west of the Mississippi and east of the Cascades. Louisville is the largest "second city", that is, it the largest city still superseded by another city in its kingdom. (Cincinnati in Ohio)
Tier Three: Augusta, Seattle, Albuquerque, Shreveport, Los Angeles, Baltimore, Detroit, Montgomery
This one of very interesting. One, that some maps have Augusta as larger than Louisville. The other is the there are maps which make Albuquerque and Shreveport larger than Los Angeles, Baltimore, Detroit and Montgomery even though main maps have them equal in size. In any case, Augusta is the largest Non-Denominational city of the south. Seattle is the largest city to actually touch the Pacific Ocean. Shreveport is most western city of the Non-Denominational region. Montgomery is the largest city to serve as headquarters to a Non-Denominational District Supervisor.
Tier Four: New York, Chicago, San Francisco, Memphis, Boston, Savannah, St Louis, Albany, Natchez, Pocatello, Eugene, Lexington
Boston and Albany (both District HQ's) are both the most Nothern cities in excess of 65,000, and no city in the eastern half of the former U.S. exceeds 25,000. Lexington completes the "Ohio triangle".
Tier Five: Pittsburgh, Fresno, Nashville, Rochester, Fresno, Santa Fe, Mobile, Dayton, Richmond, Trenton, Toledo, Vancouver, Columbus, Vicksburg, Fayetteville, Casper, Evansville, Columbia
Santa Fe is the most eastern city of western America. Vancouver is the largest city in what was Canada, and (presumably), the most northern city in excess of 25,000. Casper is the most isolated city on the continent, the farthest from any city in excess of 25,000. Conversely, Trenton seems to the closest city to another city, at least in the 25,000 range, though there's an unnamed city below Philadelphia, possibly Wilmington. In any case, this area is probably the most urbanized on the continent.
Tier Six: Washington DC, Toronto, Washington, El Paso, Providence, Little Rock, Baton Rouge, Charleston, Monroe, Milwaukee, Grand Rapids, Victoria, Bakersfield, Provo, Stockton, Knoxville, Chillicothe, Alexandria, Terra Haute, Syraceuse, New Haven, Boise, Salem, Tacoma