Thanks for your question and picture. There are a few basic principles we need to understand first:
1. Bra sizes are not standardised and change from brand to brand eg. Victoria's Secret make bras that are one cup size larger than other brands (presumably for marketing purposes).
2. Current breast tissue volume is also important in determining final cup size. eg. if you are an A cup wanting to go to a D cup, you need a bigger sized breast implant cf someone else who wants to go from a B to a D cup. Generally speaking as a "rule of thumb" a cup size varies between 150-200g.
3. The shape and size of your rib cage also has a profound influence on what size and shape breast implant you require. Think of the rib cage as a rectangular table and dinner plates placed side by side acting as breasts. The placement of the dinner plates (aka breasts) on the table determines overall aesthetics. If they are placed too far apart, looks unnatural (no cleavage). And vice versa, too close where the plates "kiss" and it looks like the "figure-8" or a bread loaf. Furthermore, if the table had a slant downwards/outwards on one half, then the dinner plate would also slant downward/outwards too. There are lots of these permutations found in women which are variants of the norm. Some women have what's called a "pidgeon chest" whilst others have a "funnel chest".
4. Breasts are "sisters" not "twins" and on many women, there are slight variations in both size, shape and contour. This asymmetry may also need to be addressed.
5. Concentrate on choosing your plastic surgeon carefully. Check for appropriate training, certification, and the ability of the plastic surgeon to achieve the results you're looking for. Ask to see lots of examples of his/her work.
Click the video above to see a lady on whom I've performed a breast augmentation who had almost no breast tissue to speak of. A round, textured implant combined with a dual-plane submuscular placement created a more harmonious, symmetrical and natural-looking result.
I hope this (and the attached video) help.