Very few designers scan and then build around a scan. Sunglasses are always challenging because everyone has a different size head, but you can get a sense of scale by either importing a 3D model of a head and making sure it is scaled to the dimensions you want, or just starting with a rough mockup in paper and measuring it to get a sense of what measurements need to go into 3D.
If you are just learning to model and have access to a 3D printer, or enough money to get it printed on a site like Shapeways, then you can build the glasses and prototype them - like any real design process you should never expect your first design to be perfect, and you’d need a lot of iterations.
In terms of getting better at your modeling skills, it will take practice. There are plenty of Rhino tutorial sites and videos out there between Google/Youtube and other sites you should be able to find plenty of inspiration.