For a non-spinning black hole, what is the only property that determines the exact size of its event horizon?

For a non-spinning black hole, the only property that determines the exact size of its event horizon is its mass. The event horizon is the boundary in spacetime around a black hole beyond which nothing, not even light, can escape its gravitational pull. For a non-spinning black hole, which is also referred to as a Schwarzschild black hole, this event horizon is perfectly spherical. The size of this spherical event horizon is precisely given by the Schwarzschild radius, which is a direct mathematical consequence of the black hole's mass. A larger mass results in a proportionally larger Schwarzschild radius and thus a larger event horizon. For instance, a non-spinning black hole with twice the mass will have an event horizon with twice the radius. This relationship highlights that for a non-spinning black hole, no other characteristics, such as electric charge or angular momentum (spin), influence the specific size of its event horizon, making mass the sole determining factor.