Hi Lucca,
I have given up on my bra size. I have, and still am, measuring with a tape but it has been the same each time... a difference of 5 inch. However I can tell by taking pictures that the girls are growing, maybe not in size but shape?
Anyway dear, I went to Victoria Secret one time but they stop at 38, I'm a 40. She didn't measure me but looked at me in the fitting room and sold me a bra 38 something with a 2 cup additional filling. This was when I first started HRT and was looking for the 2 cup additional filling.
Later after a friend gave me two (duh) prostheses, I went to Nordstrom for a fitting where I got a 40C, but that's a mastectomy bra with pockets for C cup prostheses.
Then I went to a local store and tried 38b, 38C, 38D and 40C. The 38C was the best fitting bra.
Since they don't sell 40A and 40B locally, I went to eBay and purchased a 40A and 40B. The 40B fits the best... but these are Chinese, so can I trust the size?
Assuming the sizes are correct and matching up, then that means, in my case, the 38C and 40B are the same and both fit comfortable. My band measure at 40 and over the breast 45 inch, which is a 5 inch difference. Has been 3 inch since my start, and been anywhere from 4.5 to 5 inch over the last 2 years. In cis-gender world that sounds like a D - DD cup right? But in our trans world that's more of a B or C cup?
Any way, a few months back I started buying a sports bra, size XL and it fits perfect. (Reebok brand)
So the day before yesterday I went to Walmart and bought a XL sports bra (off brand) and it choked me to death, so to speak, exchanged it for a XXL.
Today I bought 4 bra's at Walmart, 2 strapless, 1 sports bra and 1 with cami (spaghetti ? ) straps. All XXL. 3 of them fits perfect and yet 1 strapless bra fits tight... very tight, I might need a bra extension.
Anyway, moral of my story... Bra's and breasts are the most complicated pretty things to measure.
Good luck on your search to find your size. Funny thing? Once you find your size, hopefully 1 year later is different again.