If you have smaller breasts, a necklace like this can still look good, just not for the same reasons. It might instead pull together an outfit or add ornament to a plain top. I'd err on the smaller side, but I wouldn't categorically avoid them for fear of drawing attention to the fact that your breasts aren't your best asset.
A couple other things I noticed: a necklace is definitely too long if it hangs below your belly button, and it is too fat (relative to the length) if it's thickness prevents it from draping gracefully over your breasts.
OK now I am going to stop with the fashion advice before readers start questioning my sexuality...
1. The Bigger The Hoops, The Bigger The Whore
2. Feminine Beauty is Highly Controllable
3. Don't Wear Sneakers
4. Be Careful About Wearing Large-Shouldered Tops