If you have a small head, you must have a small sizes of faces. So, when you choose sunglasses, you shall try to choose some small sunglasses. If you are not round face, you can try john Lennon's small black round sunglasses. Perhaps, it will make you look cool. Or, you can also try some small sized black wayfarer sunglasses if you happen to have a round face. Hope this help you.
Yes, choosing sunglasses needs to consider your head. But this is just one aspect. We must be integrated into all factors. It mainly contains, dressing, hair style, the shape of face, skin color, and so on. Typically, if you have a small head, usually you cannot choose too big sunglasses, but also the styles that are too small. This will be specially emphasis on the size of your head. We recommend that you select moderate sunglasses, and the color you decide.
As you said that you have a small head, I think cat sunglasses can be suitable for you. Cat eye sunglasses are the good ones which can flatter people with small faces. Aviator sunglasses are also another good choice which can help you. But you should know some rules about choosing sunglasses. For example, some dedicate sun glasses, round shape ones would be ok for a small head, and you also need to think about the face shape, such as round or thin, long face or not,. if you have a round face round shape sun glasses is not suitable.
As you said, your head is not big, so my advice is that you should keep long hair to make your head seem bigger. In the mean time, as to your question, what sunglasses look good on you, what I can say is that do not wear large sunglasses which would make you look hilarious. If you wanna find your type of sunglasses, the best way is to find them out on your own, in a large glasses store.