Interestingly, I can find both very thin and extremely obese women attractive, and it depends a lot on personality and attitude. I've met women who were barely chubby, and I thought that more weigh wouldn't look good on them, and I also met women who were twice the weight which would have been considered average given their height, and still kept fantasizing that they would look even better had they gained some more.
So, the limits for me are not set into stone, like "I would break up if she crossed that number", it's more like a strong preference instead.
And it seems to have grown over time.
Several years ago, the "ideal size" would be a girl who just barely passed the point where a clever clothing choice and careful body posture could no longer hide the fact that she's overweight. The "maximum size" would have been where she would just have to start adjusting her lifestyle because she started getting too big for some things (like furniture) as they are usually not designed for her size, and where the first limitations of what physical activities she was no longer fit for, would start to appear. Basically what most would say the border between BBW and SSBBW is.
Now, it has slowly but steadily changed so that what earlier was the "maximum size", started to become the preferred ideal size, and the absolute maximum moved out to a size, above which life would start to get really uncomfortable (difficulty fitting through narrower doorways, difficulty climbing up more than one flight of stairs, inability to fit inside smaller cars)
7 years