i have a suggestion, find a celebrity that kind of has the same shape face as you and then search them on google images or yahoo or whatever and find a picture of them with sunglasses on then chances are, those sunglasses would look good on you!
A round face has no angels and the same proportions in the width and length. So people with a round face should choose sunglasses that will make the face appear thinner and longer. They can try angular narrow eyeglass frames, such as a rectangular shape which will match the round face very well.
Angular sunglasses are suitable for a round face. So you can look for wider frames with quare or rectangular styles. Sunglasses with brow bars can also fit your face fine. And If you have a large face, you should avoid tiny sunglasses.