Sooo many factors play into the equation about breast size and it is unique to each person. Factors included are your unique genetics, the age you started hrt, family history, types of hrt, and frequency of certain hrt.
Some say your breast will be the same size as your mother and sister but honestly, I only know a small few who have accomplished this on HRT alone. Some say it's one cup smaller than your mother or sister. There's a lotta "guidance" out there.
The general rule of thumb is to take hrt and just see what happens.
For me, I am bigger than my mother but smaller than my sisters. My mom has a C. I grew almost D through hrt and my sisters both have DD.
Me after two years of HRT and no progesterone:

When it comes to breasts, your mileage will vary.