Galaxies and supermassive black holes are distinct entities that coexist within the universe, and the formation of supermassive black holes is not directly linked to the size of galaxies. In fact, supermassive black holes are thought to reside at the centers of most, if not all, large galaxies, regardless of their size.
The process of supermassive black hole formation is not fully understood, but it is believed to involve the accumulation of mass through a combination of factors, such as accretion of gas and mergers with other black holes. While the exact mechanisms are still being studied, it is generally thought that supermassive black holes grow over time, and their growth is not solely limited by the size of the host galaxy.
Galaxies, on the other hand, can come in various sizes, ranging from small dwarf galaxies to massive elliptical galaxies. The size of a galaxy is typically characterized by its stellar mass, which refers to the total mass of stars within the galaxy. The largest galaxies, known as giant elliptical galaxies, can contain trillions of stars and have stellar masses on the order of hundreds of billions to a few trillion times the mass of our Sun.
There is no specific size limit for galaxies beyond which they turn into supermassive black holes. Instead, it appears that supermassive black holes are intimately linked to the formation and evolution of galaxies. The presence of a supermassive black hole at the center of a galaxy is believed to play a role in regulating the growth of the galaxy itself through feedback processes, where the energy and radiation emitted by the black hole can influence star formation and the surrounding gas.
To summarize, galaxies and supermassive black holes are separate entities, and the size of a galaxy does not determine whether or not it hosts a supermassive black hole. The formation and growth of supermassive black holes are complex processes that involve various factors, and they can exist in galaxies of different sizes.