What is the smallest diameter black hole that can be formed naturally? I was watching the video I Put a 1mm Size Black Hole Next to Earth and This Happened - Universe Sandbox 2 discussing what would happen if a 1 millimeter black hole is orbiting the earth, and I began questioning myself, is it really possible to naturally have such a small black hole?

What is the smallest size that a black hole can form naturally in real life?
The mass at which a star is able to condense into a black hole upon its death is called the Tolman–Oppenheimer–Volkoff Limit, which is roughly 2.17 solar masses. At this mass, the Schwarzschild radius of the black hole (the size of its event horizon) is about 6410 meters by the equation 

$$r_s = \frac{2GM}{c^2}.$$

EDIT: I'd also like to add that the theoretical minimum mass for a black hole is the Planck mass, $2.2\times 10^{−8}$ kg, which would yield a radius of $3.267\times 10^{-35}$ meters, though these would not be formed by stellar collapse, though they could have formed as primordial black holes in the very early universe.