Sensor size is the physical dimensions of the imaging chip. Larger sensors gather more light, producing better low-light images, shallower depth of field, and more dynamic range.
Smartphone sensor sizes (largest to smallest): 1-inch (Sony Xperia, Xiaomi 15 Ultra): exceptional, near compact-camera quality. 1/1.28": flagship main sensors (Pixel 9 Pro, S24 Ultra). 1/1.78": flagship telephoto, premium ultrawides. 1/2.5": mid-range mains, budget telephotos. 1/3.6": budget phone main, secondary cameras.
Dedicated cameras: Micro Four Thirds (smaller), APS-C (mid-tier mirrorless), Full-frame (35mm, professional), Medium Format (largest, $5000+ cameras).
Sensor area scales as the square of dimension — a "1-inch" sensor is ~4× the area of 1/2-inch.