Quite a bit of it will depend on how your frame looks.
Simply picking a cup size out of the air may or may not be the best thing for you.
I have had BA with Dr. Z. when I had my facial work done. At the time I was thinking something in the range of a C cup. I'm 5'10 and about 195 (at the time).
Dr. Z. recommended a large D cup for my frame size. Frame size, according to the doctor, is based on a number of different elements. Height, weight and size and diameter of thorax. When he told me that I would look good as a D I was mortified.
I had in my mind the idea that I would look ridiculous. I went home and made some breast forms from rice and nylons in the size that the doctor recommended. I spent the next few days walking around with them and after a certain period of adjustment, I felt very comfortable with that cup size and I felt that it gave me the right proportions for my feminized body. Dr. Z. said that he would adhere to my wishes and if I wanted a C cup he would do as I asked, but he said that many of the other women he augmented smaller than his recommendations did have some buyers remorse. I did go with the doctors recommendation and have been quite satisfied as a result. Even though I had to go and buy all new bras...

What I'm getting at is that you should discuss the idea with the doctor and see what, if anything, she may recommend. If her recommendation seems a little high, give it a shot with some breast forms and see how you look. The numbers and cup sizes may seem daunting, but once you have your clothes on it may really make you look better.
After all, isn't this all about self esteem? This really isn't that much of a vanity issue.
I know Dr. McGinn to be very conscientious and she will give you a well thought out answer.
And as to whether or not to do it at all, I would throw my $0.02 in as "GO FOR IT!" If you don't you may regret not taking the opportunity when you had it.
-Sandy