After defining the building envelope, select a suitably sized mechanical system. If you do not need to pay to install, operate, and maintain a larger system then why bother.

Airtightness is measured by ACH @ 50 Pa, ELA (Equivalent Leakage Area), & NLA (Normalized Leakage Area). The ACH is affected by the SV ratio (surface area to volume), air barrier system and location, building envelope, and any penetrations thru the building envelope.