Women in my family have decently large breasts, and I would say my breasts are small for my frame (I couldn’t fill an A-cup in my band size, for example), but large enough that other transfems in my life have expressed jealousy, and I get men staring at my breasts sometimes - so … I think this tracks the typical wisdom I hear that you will usually be a size or two down from the typical size in your family.

The difference between my band size and my bust size is 7 inches (17.8 cm), which technically means I have DDDD / G sized breasts, but that’s using the mainstream cup sizing chart, which assumes an average band size around 34 inches (86.3 cm):

https://en.wikipedia.org/wiki/Bra_size#Cup_measurement_methods

In reality, for my actual band size, I struggle to fill even an A cup. It’s my damn rib-cage, which is very wide and makes me barrel-chested.