How to choose the best Breast size?

So I just went in for my BA consultation on Friday. I paid my deposit & scheduled my surgery 6 months out.( Feb 2018) I wanted to give myself enough time to save the money & choose a size I am confident about. I am a little confused about what size I want to be. I have wanted big breast since I was 13 and I never developed like my other girl friends.  I have always been a 30 A but having 4 children has filled them out a bit. The surgeon measured me at 300-325 CCs (a small B). I am torn between a small D or a full D. I know it's only a few CCs difference but I don't want to go too small or too large. I chose 425 but I may change to 450. Can anyone tell me how they were able to choose their size. I am 5'6" 120lbs. I am very athletics with long lean legs. I have a big butt but no boobs to balance it out. Lol I don't want to go too small & regret it or go too large & be unhappy. I am so confused!