* Andromeda has a supermassive black hole, not a black hole galaxy: Galaxies don't contain "black hole galaxies" within them. Galaxies are vast collections of stars, gas, dust, and dark matter. At the center of most galaxies, including Andromeda, is a supermassive black hole.

* Supermassive black holes are difficult to measure directly: We can't directly observe black holes because they emit no light. We infer their presence by studying the effects of their immense gravity on surrounding matter.

* The diameter of a black hole isn't straightforward: The "diameter" of a black hole usually refers to its event horizon, the point of no return where gravity is so strong that not even light can escape. This is a theoretical boundary, not a physical object.

What we know about Andromeda's supermassive black hole:

* It's called P2: This is the name given to the supermassive black hole at the center of Andromeda.

* It's massive: Estimates suggest P2 has a mass about 110 million times that of our Sun.

* Its size is related to its mass: The event horizon's size is directly proportional to the black hole's mass. Using the mass estimate, we can calculate an approximate event horizon diameter for P2.

Important Note: The actual diameter of P2's event horizon is not precisely known due to the difficulty in measuring black hole properties. The figure of 110 million solar masses is an estimate, and more precise measurements may change the calculated diameter.

In summary: You can't speak of the "diameter of a black hole galaxy Andromeda" because Andromeda is a galaxy, not a black hole. The supermassive black hole at its center, P2, has a substantial size, but its exact diameter is subject to ongoing research and estimations.