I have a miniscule town in Mississippi's Pine Belt. My original population estimate was 666, because I have no sense of subtlety. I conceptualized this town as being the sort of place where Walmart is the town square, having long since destroyed anything resembling a local business, but upon research discovered that towns with three-digit populations tend not to have Walmarts (particularly not the Supercenters I was envisioning) -- the smallest to have one was a place in West Virginia with 411 people, but that store is closing this year alongside practically everything in comparable towns.
I need the smallest population that could sustain a Walmart Supercenter. I really don't want to have characters forced to drive to a larger town for a Walmart, but I also need something small enough to hammer in that this is barely more than a speck of dust (and that Walmart's hegemony has only worsened that). Complicating matters, I'm from Australia and the numbers that Americans throw out as being tiny towns don't necessarily match my perception of one -- 20,000 people is widely referred to as a small town, but that's the second largest city in my home state.