I wear forms for gender-bending, so it's not the same thing, but I wanted a modest yet clearly feminine chest. (Drag isn't exactly my style.)
Well, I can go from from really obvious to "wait, those are big pecs, right?" or even completely hidden depending on cut, pattern, and thickness of clothing, with a fairly typical 11:10 bust-to-underbust ratio, 37":33.5". Or, a better image, they fill about half the space inside my crossed arms.
My size (both on charts and in fact) is 34D.
For strange reasons I don't entirely understand,
85% of women wear the wrong bra size - nearly always a loose band and small cups.
Put another way, cup sizes are smaller than they have a reputation for.
I'd guess many cis-men are A-B, large chests with a little bit of breast development. Something like 44A, a 10.23:10 ratio.
Just a suggestion but whatever size you end up with, take an open mind to a professional fitting. Even forms are uncomfortable in the wrong size (34C is a big NO for me. *shudder*). I can't imagine how much a poor fit would suck with actual breast tissue.
EDIT, PS:
D'oh! Why?
For the same reason I don't shave my for arms or wear blue sparkly eyeshadow or huge heels or have any interest in hoop earrings. I personally feel that there is such a thing as trying too hard, where one feature can distract from the whole.
On top of that (and this is just me) I feel rather gender-neutral inside and going too far presenting either way doesn't feel right.