What size breast implants would suit me?

I'm 5 foot 6, and 120 pounds. Currently my breast size is a 36 A. Lately I have been really considering a breast augmentation to be fuller, and have more cleavage. What breast size would be more ideal for my body type? I was thinking of a 36 DD, but I know I'll probably have back problems if I go too big. Is 36 DD too ambitious? Thanks in advance!