To me, long necklaces look the best on small breasted ladies. Think the 1920s skinny flapper look. The long necklace looks amazing long and swings. I am 5’ 2”, 108lbs, and also small breasted, and I wear the 20 motif which is 34”. My actual favorite length for a long necklace is 38”. I also wear long Mikimoto pearl strands that are 34”, 38”, and 52”.
To me, long strands look great when they land around the torso, it gives a very elongated look.
I think the 20 will be amazing on you. You have the perfect body type for it. Plus, it is a nice bold statement piece that pops against tops.
I really do hope you will love the 20. Good luck!