I know this title seems that my post will be complicated but actually it is pretty easy.
I want to return to my favourite topic: Black holes, and their size.
You probably know that inside black hole is singularity and that the edge is called event horizon. Its size is different from one black hole to another and we can use Schwarzschild radius (SR) to count it.
Schwarzschild radius is area from which light could not escape if you pressed this mass to area of sphere with Schwarzschild radius.
Its equation is:
r=2Gm/c^2 where r is radius, G is gravitational constant, m is mass of object, c is speed of light.
Earth´s SR is 9mm and Sun´s is 3km and if you would compress mass of observable Universe you would have SR of 13,7 billions of light years and super massive black hole in the middle of Milky way has Schwarzshild radius 7,8 millions of km which is also how big is its event horizon.