It just depends on the shape of your face and the shape of your sunglasses. If you have a smaller face, you should choose sunglasses that will make your face look bigger. If you have a bigger face, then you should choose sunglasses that can make your face smaller.
I think a pair of thin half rim sunglasses will make your face appear thinner because half rim glasses have no frame on the bottom, which will make your face longer, so your face will appear thinner with your glasses on.
It should depend on what type of your face and your glasses are but it has nothing to do with whether it is full frame or half frame. Generally smaller sunglasses are best for smaller faces; larger sunglasses for larger faces. The frame shape should contrast with the shape of the face.