Minimum Viable Size for a Black Hole?

[Thomas Cardew]

What's the meaning of the Planck mass? I understand that the Planck length is the smallest possible lenght, but that mass is relatively huge, neutrinos are much smaller, and photons allegedly have no rest mass at all.

Planck units are units derived from fundamental constants: gravtional (G), speed of light in a vaccum (c), Planck (h), and Boltzmann (k_b) normalized to 1. They have the advatange of being 'natural' units rather than human defined. The kg was originally a defined standard, a human designated amount of material, if you use the Planck definition of mass m_p=Sqrt( (h*c)/G)) everything is in terms of physical constants and thus less changeable/ subject to error.

The Planck mass works out to be relatively huge in terms of sub-atomic particles.

EDIT: Or rather, subatomic particles happen to REALLY small in terms of the 'natural' scale of mass.