Does the event horizon of a black hole increase or decrease by adding mass?

The radius of the event horizon ($r_mathrm{s}$) is directly proportional to the mass of the black hole (M). More exactly:

$$r_mathrm{s} = frac{2 G M}{c^2}$$

The black holes whose merger was detected by LIGO would each have been about 90 km in radius, and after merger, a little less than 180km.

Read about the Schwarzchild radius