Take a look at how I did it for my DNA ring design. That's one way, just put links in your description. The description box uses Rich Text format. Here's a Rich Text online editor that I use. Another similar method is to use the Product Variants Mod. Shapeways is still under development so they haven't implemented anything easy yet like Etsy's drop down lists for selling different sizes.
You can also look at my shop and see if this is how you want to set up your shop using various ring sizes. I actually made a tutorial because I found that the information on this was little. https://www.shapeways.com/forum/index.php?t=msg&goto=924 90&#msg_92490 But first check my shop to see if it is what you want because there are different ways to do it. Cheers, Mike