It seems that the first step is to approximate the radius using Newton's Method.
I think this comes out to a ~1.12344e13 km radius (1.187 light year) using a photon traveling c with an angular momentum of 4m/s. This would equate to a diameter of 150,193.78AU (2.375 light years). The black hole needed to create this horizon would be 7.5646E42 kg, which is a bit bigger than the mass of the Milky Way at 6E42 kg.
The observable universe has a Schwarzschild radius of approximately 10 billion light years, and the Milky Way would have a ~.9419 ly radius, which means that such a black hole would be larger than the supermasssive black hole at the center of the Milky way plus all the matter (~6E42 kg).