Here is another way to quantify the size of the US economy (From US Mayors.org via CityLab).
Economies are measured using Gross Domestic Product (GDP)---the monetary (dollar) value of the production of goods and services within the borders of a country in a given year. That total dollar amount is derived from each the individual 50 states Gross State Product (GSP). In turn, each states GSP comes from the sum of production from all its cities, towns, villages, etc.
The Census Bureau divides the country up into "Metro Statistical Areas" or MSA's. MSA's could be all contained within a state OR they could be the combining of the population areas of a multiple of states. For economic and social purposes you could consider these MSA's separate and distinct from the areas around them because of their concentration of people, commercial and social activities.
To get an idea of the economic impact these MSA's have, this report compares their economic output with countries around the world.
For example, the MSA of the New York, NJ, Pennsylvania has a GMP of $1.335 trillion dollars. If this MSA were its own country, it would rank as the 13th largest economy in the world (below Australia, above Spain).
Here are the Top 90. Go to the report to see the whole list and find your MSA or country and see how it ranks!