They adapted a passage from each of them to come up with nine versions for each novel, from an underweight heroine with high body esteem to an overweight one with low body esteem, so there might be a character who states: "I'm 5'4", 140lb, and a size six", or one who says: "I'm 5'4", 105lb, and a size zero".

They then distributed the passages amongst 159 female university students, who after reading them were then asked to rate how they felt about various body parts and sexual attractiveness. The study found that when the narrative was about a slim heroine, participants felt "significantly" less sexually attractive, and that when it featured a protagonist with low body esteem, readers were "significantly more concerned about their weight" than participants in the control condition.
I think it's a bit of a logical leap to say that this harms body image long-term, but it certainly seems to raise awareness of body image in the short-term, and can exacerbate existing body image issues (which we know are distorted from all sorts of other sources).