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Magnification is a pretty meaningless concept in this situation. First of all, you are not magnifying, you are diminishing. The size of the Andromeda galaxy at prime focus is measured in millimetres. The real thing is 100000+ light years across. It's not like taking a picture of a cockroach and pinning a real cockroach to the pic and comparing the two. Even comparing angular sizes is a bit tricky. If you hold a picture 20cm away. it's angular size is twice as big as if you hold it 40cm away. If you print it on A3, it's twice the size of an A4 print, so is the magnification twice as much? You can go round and round chasing your tail here trying to make sense of it all.
However, here is a rough guide. Print the pic on A4 paper and hold it about 30cm away. If the long side has field of view of x degrees, then the image looks about 50/x times bigger in angular size than if you looked at it in the sky with the naked eye. So if you have a half degree field, things look about 100 times bigger than with the naked eye.
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