The proper method is size to the cooling needs and let the backup make up the heating. Around here, we sometimes have twice the heat needs than cooling. Many of our 2.5 ton pumps have 20kw backup to make up 30,000 BTU diff in bitter cold weather. Our all electric heat pump homes have cheaper heat than those with gas heat. If you oversize in cooling, you end up humid & clammy. That would make you miserable all summer long.
Without seeing the house and running my own figures, hard to say but I don't think he'd undersize you. He'd get nothing but grief. I'd trust him and not worry about it unless you can't cool in a heat wave.