You know vintage and exaggeration is the fashion of sunglasses just now. However, the small face will have some limit in the choosing of sunglasses. The exaggeration and full decoration will not suitable for you. The one you need is an exquisite and classical vintage sunglasses with some tiny ornaments. Otherwise, your face will seem smaller than it's real. Simple but elegance style is the best choose for you, no matter what brand or color. Ok, go ahead to pick for yourself, you can try some colorful ones to light your image. I can give you a small suggestion, if your skin color is light or blonde, the red tones will let you in pink face with good complexion. Hope the suggestion will help you!
Ok, from what you said, I can see that you got a small face and in my opinion, you are not suit for those aviator sunglasses or over sized sunglasses. Anyway, there are some sunglasses designed for ladies which could make you appear to be awesome while be compatible with your face. Or cat eye sunglasses, which could be perfect with ladies.
If you want to buy the sunglasses to match with your small face to show the fashionable style, you could take the channel sunglasses into consideration. This brand of sunglasses is always designed in so feminine character which brings the so fashionable style. You could go to the real store of eyeglasses to have a look.