Hi Jennifer,
Finally a subject I know something about! I've got at least 4 sets, and they reflect my selection criteria of wanting to pass over time. Firstly, I've always ended up too small. With large shoulders I think you need some volume, so I'd recommend D cups. In saying this, my C cups get warn the most and this is what I sleep in. A set of A cups are fun and I wear these all the time to the shops to stay in touch with my female self, without nipples they also work well inside the padded panties to shape my bum.
The B cups were thrown out in a purge moment, but they were useless. Limited shape. So starting with C cups, I felt the need to go to Ds, a common problem for a lot of women I think!
Since this is your first set, go for C cups but you will want a larger set to fill out.
A word of warning, they are addictive and for me took me deeper into femine mode. Also, I wear a diamond engagement ring, which tore the implants. Some silicon sealant coated overtop of the cut seemed to fix it.
Also, big size on the top equals bottom as well. So the bigger you go the more padding you need on the hips. This is why I'd stop at D's, shoulders are a bit hidden but I can stay slender, which is my female objective.
Sophia