If someone was above the galaxy, how much of it can he see? If someone was far enough above the milky way to see the entire diameter, how much can he actually see with naked eye? I'm aware that on Earth, we only see stars within 1,000 light-years (roughly) by naked eyes. I found myself wondering if someone is more than 1,000 light years away from milky way then we won't be able to see it?
The Andromeda galaxy is not too different from ours, and yet you can see it with your naked eyes at 2.5 million light years distance. You do need to be under a very dark sky, far from cities, and it does look like a mere white smudge, but it is visible.

Now compare that with the fact that the Milky Way itself is quite visible to the naked eye if you're far from cities...

How do I see the milky way?

...and basically interpolate somewhere in between "little white smudge" and "river of stars filling up half the sky" to get an idea of how our galaxy would look like from its vicinity.

It would not be bright anyway, and you would have to adapt your eyes to complete darkness first. But you would definitely see its outline. Whether you would distinguish anything of the shape of its arms or not, is harder to say, and would probably depend a lot on the actual distance.