This isn't a question that makes sense. I know women who are a size 6 and would look like whales if they were size 12, and women who are size 12 who would look horrifyingly gaunt if they were size 6. When the frames that women have vary so much, the idea of an 'ideal' is absurd because what's ideal for some would be totally disproportionate for others.
I can stretch to saying that usually I think of anything size 14 or above as a bit too big, but I couldn't go much further than that.