I realize this really is a matter of opinion, but I want to get some other points of view on the subject. The most interesting aspect to me is that someone from Dallas, Houston, Atlanta , or Miami certainly believes they are in a "big city", and would consider places like Charlotte, New Orleans, San Antonio, or Austin as "small". Yet, people from New York City, Chicago, or Los Angeles would consider them all "small". I have personally come up with this way to categorize large cities; when a metro surpasses 2 million you can be considered a "big city", when a metro surpasses 5 million you can be considered a "metropolis", and when a metro surpasses the 10 million mark you become a "megapolis", "giant", or whatever other name you can think up. Because places that large are few and far between, and really cannot be compared to 99% of cities.