D247's was a 4mm for sure....I saw it in person.

LOL! Thanks DM3, sounds like you know my jewelry better than I do! Thanks for keeping me honest girls!

I would go 4mm. It will balance well with the larger stone size and for not much more money, will give you a nice, sturdy, long-wearing shank.

Regardless of which Roseco you go for (3 or 4mm) you will get a seriously substantial setting. They are not in any way shape or form flimsy. It will come down to how thick of a shank you like visually.