I would be using this service for documenting structure fires as part of origin and cause investigations (single family homes, warehouses, etc… ) not large acreage. So my question is how small an area can be mapped?
Houses and warehouses should be big enough. I guess if it gets too small it might be better to just take a single picture.
You could use JOSM’s measurement plugin, or hack JOSM source to let you zoom closer. But there wouldn’t be much point to it:
*The OSM database stores coordinates in a format that has a 1.7cm granularity depending on where you are on earth (longitude granularity does get finer near the poles). So you can’t store anything more precise in OSM.
*Plate tectonics cause movements of more than 1cm/year in most places of the world, so while you can map something with cm accuracy relative to itself, the absolute position (which is what OSM stores) would need to be updated every year.