It probably depends on how you are measuring size, but I've always heard Atlanta's Hartsfield-Jackson airport is the largest. I think most cities of this size have two airports, but for political reasons, it was decided a long time ago only to have one in Atlanta. Also, there are a ton of surrounding areas that have to use the airport there.
From their website:
"The Atlanta Airport has the largest passenger terminal complex in the world."
From wikipedia:
The following airports make claims based on objective volume measures that are defined above:
Hartsfield-Jackson Atlanta International Airport, Atlanta, Georgia, United States:
Largest number of passengers annually. (1998–2008)[5]
Most arrivals and departures (operations) annually. (1999–2000, 2005–2007).[6] In 2007, Hartsfield again set the world's record for most aircraft movements in one year with 994,346.[7]
Most flights (2006).[8]
