Personally, I invested $50 in HVAC-Calc, did my own Manual-J, then turned away all the contractors who either (a) did not want to do their own Manual-J, (b) told me I needed a bigger unit despite my calculations, "just in case."
The contractor I chose actually recommended the smaller system, and guaranteed it would meet my expectations or they would install a bigger one. He measured my ducts and told me that they simply were not built to handle more airflow.
Long story short, I got a 2.5 ton system in a 1,800 sq. ft house with high ceilings. On very hot days the system runs continuously and cools to about 74 degrees, but taking all the humidity out. It actually feels chilly and we usually turn it up to 76 or even 78.
I was amazed at the number of contractors who showed up, asked me the square footage of my house, and without doing any measurements or calculcations said I needed a 3.5 ton system. I showed them the door immediately.