The distance between Earth and Stephenson 2-18 is 19,570 light years (1.15×10¹⁷ miles). The thing that makes this reasonable is because the Earth is in the Milky Way Galaxy and Stephenson 2-18 is in the Andromeda galaxy.
Stephenson 2-18 is in the Milky Way.
It is located about 19000 light years away, but this value is very uncertain. There is a cluster of stars which is somewhere between 25000 light-years and 16500 light-years (with a best estimate of 19000) The milky way has a diameter of about 100000 light-years, so this is still very much inside the milky way.
It is not known for sure if Stephenson 2-18 is a member of this cluster, it could be a foreground star, in front of the cluster (from our perspective). This means that the star could be between 25000 light years and 10000 light-years.
The difficulty in determining its distance is because it is too far to get a good measurement of parallax. Instead its distance is estimated from it's motion around the galaxy relative to Earth. As the distance is very uncertain, its absolute brightness, and hence size are also uncertain.
It is certainly not in the Andromeda Galaxy. It is in the constellation Scutum, which is not even in the same direction as the Andromeda galaxy. Moreover the Andromeda galaxy is about 2.5 million light years, one hundred times further than this star.