No, disagree. If she would have said nobody can wear them your point would be valid.
What she said was unless you're a size zero or two, you will look fat. That draws a line quite clearly. If you're size three or four, you have to dress defensively or you will look fat. And the boys don't need to be given "permission" by her to call people fat either.
Now, I don't think she should be fired or anything, but it's clearly not a good thing to say. It could have been worded like this "leggings just aren't very attractive on most people, so carefully consider if you want to wear them."
Still might be a bit much, but at least not specifically calling out people by the size of clothes they wear.