Ok, so I made some hard tests, because just spitting numbers is not enough. The complexity of the room shape and the amount and positions of its windows are more important then the raw size of the building.
For these tests, I made a 7x7 (5x5 inside) with walls covered in windows. Got cramped anyway. (Probably because the 3 wide door requires a 1 space around it, which couldn’t be used to place windows, making that side less “open”)
A 8x8 (6x6 inside) full of windows got neutral, no thoughts about it. Here’s an image:
Then a 9x9 (7x7 inside) full of windows already gave the spacious thought! Image:
So, at the end, simple don’t bother with it, at least not to the point of changing your designs around it. After all, it is just a few seconds with that thought. When they leave the house it is just a matter of a few seconds before they update their thoughts and forget about how cramped or spacious it was.