How small can these sensors get, and does size matter?

The latest mini sensors (Tapo T110, Aqara T1, Aqara P2) are roughly the size of a USB stick or smaller, which specialist reviewers repeatedly call out as a real install benefit on narrow window frames and modern flush doors. Bulkier units like the SONOFF SenseGuard Gen2 and older eufy/ecobee models look more obvious but tend to use easier-to-replace batteries.