The city seems a small town rather than a megalopolis or something. For sure, the city has one nuclear power plant, an elementary school whose principal is Seymour Skinner and a bar run by Moe.
The location of Springfield is a long-running joke in the series, based in the fact that nearly every state in America has at least one community with that name. In addition, Matt Groening, the creator of the Simpsons, grew up in Eugene, Oregon. From Groening's childhood perspective, Springfield was "the next town over". Groening also lived in Portland, Oregon and one can see the connection between Portland and characters in the series - Terwilliger Street (Sideshow Bob), Lovejoy Street and Park (Rev. Lovejoy), one of the founders of Portland, and Northeast Flanders St. where the signs say "NE Flanders St." Some people have vandalized them to say "NED Flanders St." (Ned Flanders).
Springfield is near a junction of five states, yet it borders the ocean; it has a canyon, a desert, and mountains. Sometimes it snows in Springfield, and the town once experienced a hurricane. The flag of Springfield's state showed (prior to a redesign contest) a Confederate Naval Jack floating on (or sinking into) a body of water in front of a sunrise reminiscent of the World War II Japanese battle flag. (The flag needed to be redesigned because they live in a "northern state.") The state bird is the potbellied sparrow; the state pasta is bowtie. While the name of the state has never been mentioned (except once, see below), Homer Simpson's driver's licence shows that the state's abbreviation is "NT". According to Producer/Director David Silverman, Springfield is in the state of North Takoma, a fictional US state. The two-letter abbreviation NT could refer to Canada's Northwest Territories, or Australia's Northern Territory, but its clear Springfield is in the U.S. U.S. flags are everywhere in Springfield, note the flag in the opening credits.
The capital of the state that Springfield is in is called, simply, Capital City. A recent episode showed jellyfish washing up on Springfield Beach, indicating that Springfield apparently has an ocean coastline, although that has not appeared previously on any maps.
Episodes frequently make fun of this contradiction by adding further conflicting descriptions, obscuring onscreen map representations, and interrupting conversational references. The revelation that Springfield was once entirely in the telephone area code 636 and that chaos erupted when half the town was switched to the new area code 939 does not clarify the situation; area code 636 is in Missouri, while 939 is in Puerto Rico. In some episodes, like the "Behind the Laughter" episode, it is told that Springfield is in northern Kentucky. There is a Springfield in Kentucky. In one episode, someone refers to "south of the border" and Bart responds "You mean Tennessee?" (Tennessee is directly south of Kentucky.) In another episode, Superintendent Chalmers refers to Springfield school as "the most dilapidated in all of Missouri", and shocks everyone (viewers and characters alike) by this disclosure, before continuing with "and that's why it was shut down and moved here, brick by brick".
The nearest neighbouring town is Shelbyville, there is a strong rivalry between the towns, dating back to a rivalry between their founders -- Jebediah Springfield and Shelbyville Manhattan. Lisa Simpson discovered that Jebediah Springfield was in fact Hans Sprungfeld, a murderous pirate and enemy of George Washington, but decided that the myth should be preserved. The town motto "a noble spirit embiggens the smallest man" is attributed to Jebediah.
See also: List of characters from The Simpsons