How big can a black hole become and how small can a black hole become?(minimum and maximum dimensions of a black hole)
Primoridial black holes can be found (hypothetically; there is no experimental evidence yet) of any small size above the Planck mass. Stellar black holes, however, cannot have a mass below the TOV limit (1.5 to 3 solar masses)
There does seem to be an upper limit1 of 50 billion solar masses. However, I suspect2 that this takes into consideration formative constraints (i.e. the constraints posed on the formation of such a BH); and does not prohibit such a black hole from existing. After all, the Schwarzschild metric certainly does not impose limits on the size of a black hole.
Note that talking about the limiting dimensions of a black hole is slightly meaningless as the dimensions change in different reference frames. It is far easier to talk about the mass of a black hole; the radius can be calculated in various frames from that information.
2. but cannot confirm yet; I will have to read the paper more thoroughly
theoretically there should be no maximum size/mass of a black hole, or you could say the maximum mass would be if it contained all the mass in the universe;
although the minimum dimension of a black hole would be a Planck length, the minimum mass of a stable black hole is 3 solar masses;
a black hole that has a mass less than 3 solar masses will evaporate, transforming itself into radiation (energy); the smaller it is, the faster it evaporates; if it is small enough, it will instantly turn into a flash of hard radiation;
if you know the mass of a black hole you can calculate its radius and if you know its radius you can calculate its mass; note that a curious thing about this equation is that it shows a stellar mass black hole having a high density, whereas a supermassive black hole having a density that decreases as the black hole increases in mass;
so you could transform anything you want, such as your pencil, into a black hole if you wanted to, and could compress it into the required size for it to become a black hole; it is just that it would immediately transform itself (evaporate) completely into a flash of hard radiation, because a pencil is less than the stable black hole mass (3 solar masses);
this is why the CERN experiment could never have created a black hole to swallow the Earth - a subatomic black hole, even one with the mass of the entire Earth, or the Sun, would evaporate before swallowing anything; there is not enough mass in our solar system to make a stable (3 solar mass) black hole;
To determine size of a black hole first you need to determine its Schwarzschild Radius using this formula Rs= (2MG)/(c^2). To determine its photo-sphere use the formula 3Rs/2 and extends from the surface of its Rs, to determine its accretion disk (PNR for matter) it is 5.5Rs and measured from the surface of its Rs threshold.