Kate,
It varies with the person, their age, and their family background when they started HRT. Your female siblings may give a clue if you observed them during puberty. Expect a cup or two sizes smaller than your relatives, but there are exceptions! Be patient!
I don't expect mine to be large do to my age but the changes in skin tone, modest breast size, and most importantly, ATTITUDE suits me just fine.
Joelene.