it's wood floor board, piano against the wall.

it's really not a big house. i hope i do not have neibours coming to complain the noise.

is it necessary that k6 is louder than k3 or k5?

If you're not 100 % sure that they won't complain, I wouldn't take the risk to buy an accoustic without a silent system or a digital piano instead. It won't be much fun to practice if you know your neighbours can hear you and are annoyed by it and that it's a matter of hours before they're banging on your wall or door ...
I have difficult neighbours who can't stand any noise. My house isn't small and it is rather sound-proof, but playing a piano that's next to a connecting wall is a no-no in my case. I opted for the digital Kawai CA65 therefore. I can't comment on differences in volume between K6 and K3.

I would say that the sonic level difference between these models is not significant. The question is whether you can play softly on any of them. If you can, you should be ok.

I would love to add this to the FAQ section.

As a rule, smaller ≠ less volume.

Typically, smaller equals narrower dynamic range centered around the middle, so less ppp and less fff.

If you have a sound problem with your neighbor, it will require the same acoustic treatments regardless of the size of the upright you get. So get the better upright so at least you will be happy.