First off, I know that a black hole can theoretically exist at any size, and I know about the Schwarzschild radius, etc. Snowflakes could also exist at just about any size, and they of course vary considerably in different environments. But there's a characteristic area, say ~1mm$^2$, associated with snowflakes; nobody ever saw a 1 m$^2$ snowflake fall from the sky.
So: is there a meaningful average or characteristic size for black holes that exist in nature? Or do we even have enough data to answer this question?
For example astronomers associate stars with the solar mass, although stars can of course be a couple orders of magnitude more/less massive. Galaxies also have a characteristic size, varying a few orders up or down from the size of our own.
My best guess (at least for a lower limit) is the size associated with the CMB temperature, as black holes smaller than this in nature should evaporate quickly, as they would Hawking-radiate more energy than gained through photon absorption. But I don't know if this is a meaningful consideration.