I'm wondering how big towns or cities got in native North America, I know the Aztecs had some big ones (how big?), but what of the Natives we think of as only living in roaming packs? Any big Iroquois, Sioux, etc cities?
Cahokia was the largest city north of Mexico during the Pre-Columbian Period. It was built by the Mississippian culture not far from modern day St. Louis and had between 6000 and 40,000 people at its peak.
/u/bubbametzia already discussed Cahokia, but I'll chime in as well.
The site of Paquime of the Casas Grandes developed what I would conceive of as a form of urbanism. Yes, Paquime is in Mexico, but it is located outside of Mesoamerica, though it does incorporate several aspects of Mesoamerican cultural traditions such as the rubber hip-ball game, the creation of large mounds (e.g. the Serpent Mound), the creation of ritual spaces incorporating cosmological principles (specifically revolving around the control of water).
There was a three tiered settlement hierarchy in the Casas Grandes tradition, which is often associated with either proto-states or complex chiefdoms. As many as 10,000 people may have lived at the site of Paquime, indicating it was most likely an urban setting.
For more info on the Casas Grandes tradition, check out the following:
Schaafsma, Curtis F., and Carroll L. Riley. The Casas Grandes World. Univ of Utah Pr, 1999.
In addition to Mississippian culture, you might also want to look into Ancestral Pueblo culture. They built many permanent towns and monuments in the Southwest United States. Notably, they built structures into the side of cliffs, like so.
More information to get you started:
[A list of other monuments] (http://en.wikipedia.org/wiki/List_of_ancient_dwellings_of_Pueblo_peoples)
Unfortunately I don't have access to more information at the moment, but hopefully someone else might be more qualified to expand.
There's a lot of wide-ranging opinions, but I've heard estimates as high as 100,000 for the mound-builders at present-day Circleville, OH - most notably from Roger G. Kennedy, former director at the National Museum of American History. His book "Hidden Cities" is very interesting if you're into the subject, and the history of guessing those numbers:
http://books.google.com/books/about/Hidden_cities.html?id=laZ1AAAAMAAJ
He also did a great talk at the Long Now Foundation that lays out a lot of those ideas, entitled "The Political History of North America from 25,000 BC to 12,000 AD":