I believe the metalmind has to be at least touchable for you to fill it, so no using metal dust, but we have not seen any metalmind smaller than can be comfortably handled. From what I remember, the medallions from south Scadrial are the smallest we have seen, or maybe TLR's earrings and rings, but the threads of metal through the bands of mourning are less volume wise, but had way more Investiture per volume than the coins or earrings. We might see more micro-metalminds in the future Eras, but those might have to be Invested via a different process, or through more skill as ferrings become more common.
This is off track, but imagine a hemalurgic spike bullet, aimed correctly, would forcibly give the person a hemalurgic ability. Or even further into the future, micro allomancy grenade bullets fueled by chromium, wiping the stores of metal without need for a leecher.