Need help with a simple Blackhole question Suppose the amount of mass in a black hole doubles. Does the event horizon change? If so, how much does it change?
Let's have a look at the Physical_properties subsection of the Wikipedia article on Black holes:

The Schwarzschild radius which is defined only for a non-rotating black hole is given there as 

$$r_S=\frac{2GM}{c^2}\approx 2.95 \frac{M}{M_{Sun}} \text{km}.$$


  The Schwarzschild radius was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.


So the Schwarzschild radius increases linearly with mass, by 2.95 kilometers for every solar mass you add, provided you add it carefully and don't spill any.

Trying to express the "size" of the event horizon for an arbitrary black hole is going to be more complicated, so I'm not going to adventure into that territory. However, @Chappo's comment suggests the dependence on mass will also be approximately linear as well.