If there were any wizards and/or witches that had house-elves in their employment, would there be at least one that would have the decency to actually build a private quarters for said elves similar to that of a butler's room?
I do.
I believe if there were SOME wizarding families that had house-elves in their employment, they would have the common decency to actually build for their house-elves their own personal quarters instead of treating them poorly.
As for the size of the elves' quarters, I can see them being as big as a Wendy house full of what house-elves would need and use, similar to a housekeeper and butler.
For those that don't know, Wendy house is a British term for a children's playhouse that is large enough for one or more children to enter first mentioned in Peter Pan.