It's different for everyone.
To me personally, it isn't a size, or a color or a designer or even a style. It's how I choose to express myself with the clothes, shoes and accessories in my closet.
I'm a Fashion Merchandising major and so many people automatically assume that means 2 things:
1) I'm not very smart
2) I always dress cute, mainly in designer clothes.
Both of which are generally wrong.
In fact, I'm pretty bright (I'm also a Journalism major at one of the best schools in the nation). And on a budget that's only a little higher than minimum wage I can't afford many name brand clothes (unless I hit up TJ Maxx). But let's be honest, 80% of my closet is Old Navy.
So many people have such a skewed perception of fashion now a days, it's not one thing or another, it's all of them, together.
Switching things up and putting them together is what I love most about fashion.
Be free to create what you want, don't let the medias (mostly) terrible stereotypes stop you from doing (and more importantly wearing!) your thang!