There are tiny listening devices used by the millitery an intelligence agency's but they are limited by range an functionality, an also battery life. You would have to find a size that has would alow for a large enough antenna to make the signal viable, have a lager enough battery and contain a pcb able to support all of the internal features like flash chips, microphone speaker ect.
Maybe something the size of a womans lipstick if you go for a coiled antenna (the coil allows for maximum lengh to space ratio) but the smaler the device you will always get lower signal quality on your microphone an speaker.