Compare the two pictures above of the same lady. She is a breast cancer survivor. As a cisgender female, she has a luxury that none of us transgender folk will have - her whole body sends out cues to tell the world that she is a woman - even without the breasts that (sadly) had to be removed because of cancer. (I hope she's a long term survivor. I can easily say that no one deserves to die from cancer, having lost a wife to that dreaded disease.)
For many of us M2F transgender folk, we often do not have a clue about breast size, and what size prosthesis we need to use. I figure that one should use a little common sense. First, you must find out what your band size would be, (I'll let you look up how to measure that.) And then the fun begins. You should then find a few internet sites that sell women's clothes, and look to find an average cup size available for that band. Once you determine the average cup size for someone with your band size, go up a cup size - and voila! you now know what size bra you need. Yet, this is only a first step in the process. You must go to an outlet such as The Breast Form Store to start shopping - and there you will find charts to tell you the size of breast form needed to fill out that bra you'll likely be buying.
Today, I received a set of swim forms which I intend to use in the hot tub on my autumn cruise. They do not project as much as I'd like. But they will do the job with I am cruising with Lili - filling out a woman's swimsuit in an appropriate way. And given their size, I probably will be able to use them after I have lost some weight, as they seem a little small - which will be not be as much of a liability as I lose weight.
Soon, I will be buying replacements for the breast forms I use today. After 3-4 years of use, it's time to replace them, and they do show their age. And yes, I'll likely go to the Breast Form Store again, as I don't want to pay the expenses of going to a nearby "mastectomy boutique" for items I can get by mail.
Hopefully, they will find a better "cure" for breast cancer - something better than removing a part of a woman's body that is a part of her identity. Until they do this, I'll be glad that these forms are available for women, as every little bit more feminine a woman can see herself, the healthier she will feel about herself and about life.