Valerie, I took into account the size of the breasts of the women in my family, my development after a full year on a high dosage of hormones, and the advice of several different doctors with experience in the area of HRT before I decided to get implants when I did. Yes, there is still a chance my breasts will grow more, and possibly more than I'd like, but the odds are I won't see much additional growth. And so far, the benefit has far outweighed the risk, as having breasts appropriate to my dimensions has made an enormous improvement in my confidence level and sense of completeness. My smaller breasts were far far too small for my size, and really didn't even look as large as man-boobs.
My surgeon bases the recommended size of implants on very careful measurements of your chest as well as existing breast tissue. He wants to give breasts that look normal and correct for your body, as well as being the safest for you in terms of prevention of possible issues. My implants were 616cc, which sadly means nothing really, as that size in someone smaller than myself would be enormous, and in someone larger would be small. I have a friend who got 700cc implants and has smaller looking breasts, because she has a much larger frame. Although my breasts are DD now, they really are perfect for my frame. I wouldn't want them any smaller, and I really don't want them any bigger either.