I have found this for you. You can have a look at http://www.igotwii.com/nintendo-wii-manual-photo-channel-page-18-19. I haven't come across such kind of questions before. But I think this website will teach you how to play this and how to change the size of the sunglasses.
It is indeed a way of saving money by using your existing lenses to match new frames .but Not all sunglass frames are capable of supporting a perscription lens. You should consider the curve of the frame, your prescription and size of the frame. Before you buy the new frame ,take a picture of it and go to the optician to ask if they can do this for you .