To, the girl who was asked, "Are you sure this is your size?" or something like that,
I want to begin by saying I am so sorry someone said that to you, you do not deserve to experience such a thing. You are beautiful, and you know your body (and what makes you feel most comfortable) more than anyone. Sometimes people believe their words are helpful, but sometimes people are just being mean. Everyone has their own perception of what someone "should" look like, and their own reflections of how they feel inside. Regardless of their intention though, only you can decide how someone's words make you feel. And you are entitled to every feeling you have.
Whether this was the first time body shaming comments have been made towards you or the twenty first, it's never right, and it never gets easier to hear. Ignorant people will continue to plague our beautiful world, and there's nothing we can do about that. Instead, what we can do is learn how to deal with them.
There is no right way to react in these situations. If you cry, that is ok. Anger is ok too. You feel disrespected, and most likely embarrassed. It's not fair. My personal favorite response used to be ignoring the comment and issuing a swift, yet hurtful rebuttal in my head. But then I tried speaking up.
During one of my experiences, (unfortunately there have been a few) when questioned on my size choice by a LuluLemon cashier, I asked, "Is it no returns?" They responded, "no, you can make a return." I looked at the worker and calmly said, "If I can return it, I don't understand why you feel the need to question me on the size. I am the one spending the money after all." He was quiet, and I felt a little bit stronger. I was sad afterwards, angry too. But proud for questioning his unnecessary judgement towards my understanding of what fit best in the dressing room.
Remember that you are the one who will wear these clothes. If someone thinks you should buy a bigger size, but you feel your best and most comfortable at a size smaller (or vice versa) trust your gut. Literally. And if you decide to leave and buy nothing, I'm sure a better outfit is waiting for you at another store (with a more respectful staff.)
I leave you with the sad reality that this may happen to you again. As I said before, ignorant people are an unfavorable addition to our society that we cannot control. But remember, regardless of how you are treated, it is how you view yourself, and how you react that determine your self worth. No one has that ability but you. So validate your emotions, but always remember to be better than the person who made you feel that way.