Went through the same thing a couple of years ago when I replaced my central air and I did ask him that same question. He said that by looking at the old unit and the size of the house, he knew what the appropriate size would be. I pursued it based on the fact that the old unit couldn't keep the house cool and he assured me that it was a cheap unit and that the new one would do the job. I can now say that he was right and the new unit not only kept the house cooler, but it also cut my summer electricity bill in half.