Ephesus was once a major Roman port city of an estimated 250,000 people — one of the largest cities in the ancient world — and the scale of the ruins reflects that.
The Library of Celsus, the site's most-photographed structure, was originally a funerary monument built in the 110s CE, not primarily a working library in the way the name suggests today.
The Great Theatre could seat an estimated 25,000 people and is still used for occasional performances — worth knowing if you want to actually sit in the seats rather than just view from outside.
Most visitors spend 2-3 hours on site; history-focused visitors often want 4+ to properly see the Terrace Houses (a separate, smaller-ticket add-on showing preserved Roman residential mosaics) alongside the main ruins.