The supermassive black hole at the center of our galaxy, known as Sagittarius A* (Sgr A*), has an estimated mass of about 4 million times that of our Sun. However, when it comes to black holes, their size refers to their event horizon—the boundary beyond which nothing can escape their gravitational pull. The event horizon of Sgr A* is relatively small, with a diameter estimated to be around 24 million kilometers (15 million miles).
In terms of other galaxies, there is evidence that supermassive black holes reside at the centers of most, if not all, galaxies. Some of these black holes are significantly more massive than Sgr A*. For instance, the black hole at the center of the galaxy Messier 87 (M87), which was imaged by the Event Horizon Telescope, has a mass estimated to be around 6.5 billion times that of our Sun. The event horizon of this black hole has a diameter of approximately 38 billion kilometers (24 billion miles), making it considerably larger than Sgr A*.
There are other galaxies known to have supermassive black holes of comparable or larger sizes than Sgr A*. Examples include the black hole in the galaxy NGC 1277, which has a mass estimated to be about 17 billion times that of the Sun, and the one in the galaxy NGC 4889, with a mass estimated to be around 21 billion times that of the Sun.
It's important to note that while the mass of a black hole is a significant measure of its size and gravitational influence, the event horizon size depends on both mass and spin. Additionally, there are ongoing scientific studies and observations to better understand the properties and sizes of supermassive black holes in various galaxies.