Possibly a reaction to teasing in the school yard? I would be asking her about that in the first instance, as that is more likely to be the stressor behind the dream.

Re the fatness/thinness issue - I have always dealt with this by talking in terms of health, and the sorts of things that all people can do to be as healthy as possible - exercising, eating good food, sleeping, relaxing, keeping in touch with friends. Yes it's good to acknowledge that people come in different shapes & sizes, but fatness/thinness is not the only dimension in this. People are also taller and shorter, male and females are shaped differently, different ethnic groups have different body shapes, and there are also people with unusual features or disabilities like missing limbs. But they are all people & no matter what they look like in the outside, we are all the same on the inside, with the same hopes and feelings. It's a really powerful opportunity to talk about bullying & harrassment - it is never cool to be mean to someone & especially not for something about who they are that they can't change. There are lots of reasons why a person may be overweight or obese, and none of them reflect on the character or worth of that person.