There is a size guide for each style on the product page. Just scroll down and click the 'Size Guide' tab to view the measurements for each Swannies collection. We recommend you compare these measurements with a pair of glasses you currently own (such as sunglasses). That way you can get an idea of what will fit you best.
All non-prescription Swannies have a 30 day satisfaction guarantee so if you do find you have purchased the wrong size, we will happily help you exchange them for the right size for free.