Resist the urge to micro manage the engagement ring process. Keep reminding yourself that he's adrift in a world (jewelry) that not only is foreign to him, but as a guy - he just doesn't get. And this thing he doesn't get is going to trigger a significant investment on his part. Only someone in serious LOVE would go through this. Ask yourself, given all that, how you'd feel.
What if tradition held that becoming engaged required you to buy him a big, flashy, chrome-studded motorcycle? After wandering in a Harley Davidson/Yamaha/Honda wilderness for days, would you want your guy fussing over the model you'd chosen for him? Certainly not. You'd want a brass band. You'd want a, "Yes, I'll marry you!", and you'd want certified, life-long heroine status for making all that happen.
So does he. That's why you've got to be smart, careful and flexible. If you have strong feelings in a Vanna K semi-mount then be certain to share them with anyone he might ask (including your ring size) and make them promise to say they just happened to pick up this information somewhere along the way...and then step back.
Unless, of course, he invites you into the process. If he does, disregard the above, visit your local Vanna K jeweler, and have the time of your life!