If you have heard that a home needs one ton of cooling for every so many square feet, you have heard the shortcut that mis-sizes a lot of air conditioners. Those rules were rough guesses from decades ago, and they ignore insulation, windows, ceiling height, ductwork, and how your house faces the Tennessee sun. The proper method is a load calculation, known in the trade as Manual J, and it is less mysterious than it sounds.
Manual J is simply an accounting of how much heat your house gains on a hot day and how much moisture it has to shed. The inputs are things a technician can see and measure in one visit: square footage, yes, but also window sizes and directions, insulation levels, air leakage, the number of occupants, and the duct system's condition. Two La Vergne homes with identical floor plans, one shaded in Woodland Hills and one facing west over open ground near the I-24 corridor, can honestly need different amounts of cooling.
Sizing matters more in our climate than in a dry one. La Vergne summers are humid, and an air conditioner removes moisture only while it runs. A correctly sized system runs long, steady cycles on a 95 degree afternoon, wringing humidity out the whole time. That is the difference between a house that is 75 and comfortable and one that is 75 and sticky.