Great question Amy3...but tough to answer. Patients come in all the time and tell me their cup size,"I wear a Vic's Sec B but a Target A... and I want to be a C cup after surgery.". Because bra manufacturers do not use a uniform "cup" size to make their bras, the cup sizes and therefore the fit of bras are different.
So I have been taking the time in the initial consultation to measure each patient's "cup" size in my office. This gives me the starting point. If I measure an A cup and the patient desires a 2 cup increase (A to C) I can judge the volume needed to achieve the goal...however, other factors, like skin elasticity and quality, breast tissue volume and quality as well as width of the chest wall are many of the ingredients I use to make sure the implant of a desired volume will fit.
I have also been using 3D imaging recently to further evaluate what looks best and which implant style fits best and achieves the desired goals.
Best advice is to locate a board certified plastic surgeon with great experience in breast augmentation (or as many as you are comfortable seeing) and have them evaluate your anatomy and determine the best sized implant for your body....because, YES, an implant that is too BIG for your anatomy WILL cause problems down the road and may lead to further surgery.
Good luck!
Dr. C
UPDATE :
Extreme plastic surgery, especially surgical misadventures in other countries, has devastating consequences. Be smart and listen to what your Board Certified Plastic Surgeon recommends.
"On September 15, 2010, Sheyla Hershey had her breast implants and most of her own breast tissue removed due to Staph infection. According to her latest blog on her ofiicial website, she still has not lost her official "title" as the woman with "The World's Biggest Enchanced Breasts", and hopes to be able to have perhaps have her breasts re-enlarged within the next "few months"."