A non-spinning black hole without charge can be analyzed using the Schwarschild metric. The Schwarzschild radius (aka event horizon), measured as the circumference divided by $2 \pi$ is directly proportional to the mass, $2GM/c^2$ , where $G$ is the gravitational constant, $M$ is the mass of the black hole, and $c$ is the speed of light. So the area of the event horizon increases as the square of the mass, but the radius increases only proportionally to the mass.
The time dilation for a non-moving object measured by a distant observer, derived from the Schwarzschild metric would be
$$\left(1-\frac{2GM}{c^2r}\right)^{.5}\,,$$where $r$ is the radius (or distance) or $$\left(1-\frac{R_{\text{S}}}{r}\right)^{.5} \, ,$$ where $R_{\text{S}}$ is the Schwarzschild radius shown above.
So at the event horizon, the time dilation is infinite regardless of the mass (and therefore size) of the black hole. But if you take a fixed distance from the Event Horizon, you would have little time-dilation when $r$ is much larger than $R_{\text{S}}$. Given your numbers, the time dilation at $r = 30 \, \mathrm{km}$ would be .983 (1.000 would be no time-dilation) for a $1 \, \mathrm{km}$ $R_{\text{S}}$ black hole. But it would be much higher $30 \, \mathrm{km}$ past the event horizon of a supermassive black hole.
