lawyer: The length of the necklace doesn’t matter, because it is only to see what needs to be covered on the sides and back of the neck, in the front, the collar bone needs to be covered, no necklace needed. It should be a chain necklace which hangs loosely (not a rigid necklace). I hope you find this a little clearer.
I didn’t think I need to add this, but just in case it isn’t clear, everything beyond the necklace, side and back, including the back and bodice, needs to be covered. Sorry if this sounds preachy. I don’t make the halachos, I just keep them.