You can look your old sunglasses to see what sizes of it. Or you can measure your face to as to how what sizes of sunglasses you need. Of course, the most directly way is trying on and find one that suit for you both in style and sizes.
The best way is to see an eye doctor and have an eye exam so that you can know the size and prescription you need. And you can try them on in the real store to find out the style that suits you best. Then you can order them online with your information you have got.
If you have poor vision and need prescription sunglasses, you shall visit eye doctor and have an full eye exam. Then, the doctor will give your sizes and prescription that used to buy sunglasses.
If you have good vision, you needn't have eye exam and need to buy plano sunglasses, Maybe you can measure your face size by yourself or your friend so as to know, what sizes of sunglasses you need.