For all intents and purposes of astronomic calculations, you can pretend it's 9mm in size, because A) the difference between a 9mm sized object that behaves like a black hole and a singularity with a 9mm event horizon is minimal and B) it's a 9mm black hole going 50km/s+, most math will result in the extinction of human kind anyway.

To be fair, we don't know what is behind those 9mm. Einstein's equations predict a singularity but afaik there is consensus that those don't hold there.