Poor assumptions about HVAC needs have caused a lot of problems with improper dehumidification and resulting mold issues. Usually with the idea that "bigger is better".Surely there are some on line HVAC load calculators which can be used? I mean you need to know the heating and cooling loads and then add a few percentages. Down here in Texas the rule of thumb is one ton (12000) BTUS of heating or cooling for each 500 square feet.
Relatively simple calculations of heating/cooling loads and required system sizes can be done using well know prescriptive references such as "Manual J".
A good reference here: https://www.energyvanguard.com/blog/hvac-design-done-right-manual-j-s-t-d/