This varies according to your style!?For example: If the person likes to larger glasses, her glasses look for a bigger, which suits her face!Try one with square corners maybe. The best thing you can do is go to a store and prove it !
The shape of the frame should be opposite to your face shape. So I think you'd better choose the square frames. Nowadays the big framed sunglasses are very popular. You can have a look online. If you have any interest, you can have a look in any local optical shops and have a try.
A round face has curved lines with the same width and length. And it has no angles. You can choose rectangular lenses or interesting angles which are not too extreme. I think this kind of lenses can go well with your face shape. As to the color of the sunglasses, I think dark color is very popular and usually it goes well with all kinds of skin colors. So you can have a try. Good luck!
Choose those sunglasses that can make your face look a little thinner and longer. I think this will go better with your image and look more flattering and suitable than other kind of sunglasses. Maybe the sunglasses that are wider at the temples are fit for your face shape. You can have a try. Good luck!
Many celebrities such as Kate Winslet, Drew Barrymore and Christina Ricci are with round faces. You can pay attention to what kind of sunglasses they are wearing. If the sunglasses look good on their face, they may look good on your face too. Usually round faces should go well with the opposite shaped sunglasses to balance out its shape, so you'd better pick square framed sunglasses.