Short answer
São Miguel is both the largest and the most populated island in the Azores, home to roughly half the archipelago's population and its main city, Ponta Delgada. It also has the main international airport. Terceira is the second most populated, followed by Pico and Faial. For context, São Miguel is about 760 km², but exact figures can be confirmed with official statistics.
General information, not legal or tax advice. Rules and amounts change: confirm with official sources.