In Washington State (way out here on the Left Coast) state law defines First Class cities as having 10,000 or more residents, Second Class Cities as having 1500 or more residents, and Towns as having less than 1500 residents, at the time of their incorporation. Thus, it would be possible for a town to be organized as a town, grow to a million residents, and remain, under state law, a town. Likewise, it would be possible for a city to become a first-class city by showing that it had more than 10,000 residents at the time of its incorpation but, through some unfortunate means, lose most of its population, and yet remain a first-class city of 100 people.
There are certain words which pertain only to towns, in Washington law. For example, the chief law enforcement officer of a town is a "marshall" which cities have chiefs of police. Calling the top cop "town marshall" has the ring of the Old West to it and, indeed, these laws were written when Washington was part of the Old West.
