This is the size if the Andromeda Galaxy in relation to our moon. Although it is not quite this detailed to the naked eye, it is still very visible from a dark sky site on a moonless night, and if using a telescope or binoculars, it would fill the entire viewfinder.
This image is a blend of two images taken on the same night, with different exposure times, using a 35mm lens.
The Moon was shot with a lot lower exposure as to not overexpose it. Andromeda was shot with a higher exposure to get in more detail, as well as being tracked using a star tracker. From there, several Andromeda images were stacked together for even more detail, then blended with the Moon image.
Lets talk size. The Moon is approximately 3500 km across, but it is also approximately 2.5 million light years away from the Moon.
Andromeda is about 140,000 light years across (the Milky Way is about 100,000 for comparison and one light year is 9.46 TRILLION km).
So, with that all being said, this formidable giant is so big, we can see it with our own eyes from farther off than what we can comprehend.
In around 5 billion years, Andromeda will eventually merge with our own galaxy. Assuming by some miracle, there are still bings on Earth then, they would see andromeda grow larger and larger in the sky until its numerous stars join ours.