Hi! I am so late to this thread, but I thought I would still reply..
You have received a lot of great thoughts, and I will add my two cents.
First, I love love love 20s! The 20 is the bomb when it comes to classic VCA Alhambra. I am 5' 2", 108lbs and petite, and I love long necklaces. I only wear long necklaces, the shortest being 34" and the longest being 52". I love the swing, and I think it makes a petite person look fabulous.
A 20 works with every neckline... scoop neck, vneck, collar, turtleneck.
This is not true of a shorter necklace such as a 10 or a 6. With these shorter necklaces, my opinion is that they only look good against skin, so you need to wear vnecks, lowcut necklines, or open collars. I almost always wear a scoop neck, hence I love long necklaces.
On thoughts of doing a 10 + 5 + 5 and the other Frankenstein combos...
For me, it does not work. The spacing of the 5 motif is tighter (e.g. there are less links in between), so when strung together and showing on the neck, it makes the necklace look very "motif heavy" due to the close spacing. Also, and this is a big NO for me, you will see the clasps. Some people don't mind the clasp showing or the off-spacing, but I am a purist, and I like to wear pieces as they are intended to be worn.
I did want to point out the spacing is different with the 5 motif in case you did not know.
Hence, I vote for a pure 20 motif. I only wear white metal, so I have the WG MOP set. I love my set.
However, if I wore YG, I would 100% get the YG MOP. That is the most gorgeous 20 combo. The MOP that VCA uses for their YG has slight pinkish overtone. It glows, and it's magnificent. My WG MOP is nice, and the MOP that VCA uses for WG has a silver overtone, and while I love my WG MOP, it in no way compares to the beauty of the YG MOP.
I would also get the matching earrings in VA size. For reference, I have the WG MOP 20, VA WG MOP earrings, and two WG MOP bracelets. I wear the bracelets as bracelets, not as necklaces.
You can wear the earrings as a set, and it will look beautiful on you.
Since you are 5' 3", you will be fine with the 20 as it is 34". You mentioned you have big boobies, but the upside is that you are short, and at 5' 3", the 20 will fall past your boobies.
What you do not want is for the 20 to end at the bottom of your boobies, like they are about to fall off a cliff. You want the chain PAST your rib cage which they should be. For big boobies and long chains, the sweet spot is below the first rib of your rib cage.
For me, I like my chains long, and my favorite length is 38", so I definitely think 34" is not going to be too long for you.
I think your choice of a 20 in YG MOP is spot on. It is my favorite metal & stone combo for Alhambra. It works for all outfits, and best of all the MOP glows against your skin.
Hope this was helpful, and good luck to you!