I understand that extremely minute black holes can have a lot of mass. For example, A Black hole with a 120 pm radius, like a Hydrogen atom, would have a mass of 3.73x10^67 kg. Is this mass already defined? or will the mass increase with size and vice versa?
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5$\begingroup$ recommend asking Astronomy SE $\endgroup$– Erin AnneCommented Jul 6 at 5:16
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The size of a black hole is its mass. When we talk about bigger or smaller black holes, we're usually comparing their mass because that's their fundamental attribute that matters the most.
However, if you want to know about linear size (or area, or volume), then things get a bit messier. A black hole is not something like a rock that has a clear boundary which could be measured with a ruler. It's more like defining where the solar system ends - different definitions abound.
Still, for black holes a very convenient definition we can use is its event horizon, which a is sphere where the gravity is so strong that nothing which crosses it can escape again; not even something moving at the speed of light. The radius of this sphere is (for uncharged, non-rotating blackholes) the Schwarzschild radius, given by: $$ R = \frac{2 G M}{c^2} $$ which, yes, does grow with mass.
Using this equation, we can see that the figures quoted in the OP are wrong. A radius of $120$pm gives a mass of about $8\times10^{16}$kg. The given mass of $10^{67}$kg is preposterously large, something like a hundred trillion times greater than the mass of the entire visible universe - despite said universe containing a lot of black holes that are a lot of bigger than $120$pm.