Sounds like a good plan - small rat every other week.
I would go by body definition.
Don't worry about the small rat vs. medium rat for her. She sounds like a good eater, so she'll probably just be excited for food and not measure it up.
Keep in mind, it may take a long time, like a year + for her body to completely change shape.
Just know you are doing good by her and that she will be fine.
I do not think you will see her drop weight at all. I think she will grow much slower and will probably grow into her current weight (range).
Good luck and keep us posted.