I just read this exert from an article:
According to NASA, that tail is 5 km long, so it might be surprising that it’s not visible to the unaided eye, but considering the closest it will get is 10 million km from earth, it’s impressive we can see anything at all.
That sounded like a lot of crap, as it will be impossible to see something 5km long from a distance such as that of the comet from the earth.
Following the link they provided, NASA said:
Comet NEOWISE has remained unexpectedly bright, so far, with its ion and dust tails found to emanate from a nucleus spanning about five kilometers across.
That is a very different statement about the significance of the “5km”. Moral of the story: don’t trust everything you read.