The fastest way to lose a customer is an oversized unit that short-cycles their house into a clammy cave. Run the load math first, walk in with a defensible tonnage, and let the competitor's "bigger is better" pitch die on its own.
Good enough to quote confidently and catch a bad bid: climate, insulation, ceiling height, sun, and occupants move the number 20–35 BTU per square foot. For the design itself, Manual J is the standard — this gets you into the right half-ton before you run it.
An oversized unit cools the air before it dehumidifies, then short-cycles: cold, sticky rooms, higher bills, shorter compressor life. Right-sized equipment runs longer cycles, wrings out humidity, and lasts. Bigger is not better; matched is better.
Check the nameplate MCA and MOCP — then verify the run with the wire size calculator (MCA already includes the 125% rule) and the voltage drop calculator if the disconnect sits far from the panel.