Notice how it would look so much larger than the moon? Now think about this. The Andromeda galaxy is around 2.5 million light years away. Imagine how large it has to be in order to look like that even from that unimaginable distance! In a way, this image gives you a sense of how large 100,000 light years (the visible part, there are invisible parts that stretches Andromeda to 220,000 light years) is, well not completely since such sizes are unfathomable, but this will do.