Normal fission can be done at very small scale. There have been plans for something light enough for a man to carry. The core mass can be very low, for example water moderated homogeneous reactors can have fuel inventory in the low kg’s. However i think the only ones fielded where isotope thermal types. There are some pretty small lab reactors IIRC with outputs in the kW range.
Regulation has more to do with perceived safety than anything else. Note that when a FF is on, it is quite a potent source of radiation without proper shielding. So there will be some regulation.
