I've designed a device that utilizes mechanical force to transmit information that was around 5nm in diameter. It was based on the human Notch receptor. It's a few hundred amino acids in length, folded to produce a protein that senses force transmission, is cleaved upon unfolding, and releases a transcription factor the nucleus of a cell.
I kind of find the distinction of 'robots' vs cells funny, as once you get down to the (sub)nanometer level one's intuition should flip: organic material acts stiffer and more lego-like than metals - which act more like unreliable putties. A "device" that becomes small enough is much more likely to be made of organic molecules than metallic molecules - cells ARE those futuristic robots...
The kinesin motor proteins are pretty cool too [1], but those are naturally occurring machines that I suspect we'll be imitating for a long time.