People with diamond shaped face just like Jennifer Aniston's, they should opt for more delicate styles, such as wire or oversize plastic frames, that won't draw attention to the forehead but draw attention to the downward. So they may need glasses that add width to the lower part of their face, so bigger styles work well but avoid anything that sits high and wide. Sunglasses with round edges look great and soften the broad upper part of the face.
Diamond-shaped faces are very rare and characterized by narrow at the eye line and jawline, and the broad cheekbones that may be high and dramatic. For different circumstances, just like, the face which is bigger can fit lots of styles. In particular, for a small face, then I suggest the smaller sunglasses frame is better. Normally, people choose rimless frames, oval and cat-eye shapes, but I think this come to secondary. For the final choice, the key is yourself, how to go.