(((hugs)))) I agree, she didn't quite do this right, did she? I can appreciate her wanting you to look your best (and I hope you can find that appreciation also.) Maybe she really looks up to you, and wants you to be the best (she thinks) you can be, and that includes how you look.
Personally, I would want someone to help me look good, but I'm not sure it was her place to do this "for" you.
Does she, herself, work at being stylish and looking her best? Is she a perfectionist? Does she always try to "help" others this way? It could be she isn't polished in suggestions yet (she is young.)
However, it must have hurt. Is it really sexual harassment though?
If her comment has merit, then take it to heart and seek out a store that has a fitter. If it doesn't have any merit, then brush it aside and try to move on.
If she didn't care, I doubt she would have said anything. Don't judge her for this, she might turn out to be a really good friend in the future.
((((hugs)))