What is the smallest Zigbee switch?
We’re gonna need a LOT more info in order to help you. Light switch, relay switch, intended use?
Door/window sensors are probably the smallest, followed by leak detectors.
These are small:
https://www.aliexpress.us/item/3256807326459098.html
https://www.aliexpress.us/item/3256807932432505.html
https://www.aliexpress.us/item/3256807208799477.html
light switch like Sonoff ZBMINI Extreme
So that device looks like a relay that you stick in the switch box. I use a lot of Shelly relays for that but you’re asking for the smallest. I assume because you’ve found shoving everything into the switch box cumbersome and want a smaller box? Perhaps just replace the switch with a zigbee switch - I have several Enbrighten zigbee switches and dimmers that work well and no problem fitting into the box.