[adreid]

Since you have to feed it ultrasonic energy to power it, I suspect it is not too bad because, IIRC, ultrasonic has a short range.

You could feed it electromagnetic energy - but then all you really have is a smaller version of the Great Seal bug https://en.wikipedia.org/wiki/The_Thing_(listening_device). I wouldn't be surprised if those already exist in about this size and the paper linked to from the web page talks about several such devices (and how they are not as well suited for implantation).