Ok, lets say we wanted to know the diameter of the largest black hole. (Its event horizon) Lets say the universe had contracted and all the galaxies had been consumed by black holes and all merged into one single massive hole.

Say there were approximately 100 billion galaxies, all relatively the same size, and each had an average of 200 billion solar masses. 1 solar mass = 1.9891 × 10^30 kilograms

I believe the formula to calculate the radius of the event horizon is

2GM/c^2
Not being very good with math, perhaps someone would be able to calculate this? Thanks in advance…