If we extend the definition of "Z80 computer" to include those which aren't using the original Zilog Z80 DIP40 CPU, e.g. SoCs, then these may be the smallest:

https://en.wikipedia.org/wiki/S1_MP3_player

https://www.bunniestudios.com/blog/?p=138

https://www.bunniestudios.com/blog/?p=139