Start with a good idea and develop a design completely your own based on the main dimensions.
Then paper prototypes are a great way to get your head around the sizes. A 138 or 140 frame width already makes a significant difference for eyewear.
If you are working with Rhino3D consider the Grasshopper plug-in to develop a scalable design according to variable input parameters. If designed well you will have an entirely parametric design and ready for 3D printing!