First, let us assume the following > M is for mass, R is for radius, D is for density and V is for volume. The radius of a thing like a black hole has proportional relationships.

The radius of a balck hole is found to be directly proportional to its mass.

(R- M)

The density of a black hole is given by its mass divided by its volume,

(D=M/V)

... and since the volume is proportional to the radius of the black hole to the power of three,

(V- R^3)

... then the density of the black hole is inversely proportional to its mass raised by the second power.

(D- M^2)

The event horizon is simply a boundary of a black hole. You can work out specific radius calculatuions, such as a killing horizon which has a radius of r=\sqrt 3/lambda. There is what is called a Schwarzschild radius - this is an area which acts like an event horizon. The characteristics are associated to the quantity of mass.

Where r is radius, G is Newtons gravitational constant, c is lightspeed the radius is found:

r=2GM/c^2