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, in terms of physical size, black holes don't have a well-defined "size" in the same way as a solid object. They are characterized by their mass, spin, and the size of their event horizon—the boundary beyond which nothing can escape their gravitational pull.
Regarding other galaxies with similarly sized black holes, many galaxies are known to host supermassive black holes at their centers. The sizes of these black holes can vary significantly. For instance, the largest known supermassive black holes are observed in giant elliptical galaxies. These black holes can have masses ranging from millions to billions of times that of the Sun. Some examples include the black hole in the galaxy Messier 87 (M87), which has a mass of about 6.5 billion solar masses, and the black hole in the galaxy NGC 1277, with a mass of approximately 17 billion solar masses.
Comparatively, the black hole at the center of our galaxy, Sgr A*, is relatively modest in terms of mass compared to the largest known supermassive black holes. However, it is still an important object for studying black hole physics due to its proximity to us, which allows for detailed observations and measurements.