What size breast implant should I get?

How do I choose the right implant size ? A, B, C, D, DD?
First of all, implants do not come in cup sizes. There is no such thing as a C cup implant, although many patients with existing implants will tell me "my surgeon put in a C cup". Cup size is relative to body size and is determined by comparing the measurement taken around the rib cage underneath the breast to the measurement around the most full part of the breast. Bra makers are all very different, and wearing a different band size (34 vs 36) can change your cup size, so a specific cup size can't really be guaranteed.

Rather than cup sizes, implants are labeled by volume (measured in milliliters). For each mL amount, there are also multiple different shapes (called profiles). Some are more flat and wide (usually called moderate profile), and some are more rounded and project more off the chest (called high profile). Each company has it's own specific way of naming implants (for example one company calls their highest profile implant "ultra high" and one calls it "full"), but the concept is the same. Also, saline implants tend to look larger and project more than silicone even when they are the same profile and mL size.

Since cup size is all relative, a 300mL implant will create a larger cup size for a petite 90 pound woman than it will for a larger framed 150 pound woman. Also, a 300mL moderate profile implant will create a smaller look than a high profile implant of the same size in mL because the dimensions (width and projection off the chest) are different for each implant even though they contain the same volume in mL.
Some surgeons allow patients to pick their size. I don't think that's in the patients' best interest. Plastic surgery requires at least 5 years of surgical training and at least 2 years of specific training in plastic surgery, so any surgeon who lets a patient choose without guidance isn't giving that patient the full benefit of their training and experience.

Choosing too large an implant can lead to nerve damage, stretch marks, distortion of the breast shape, early ptosis (sag), lowering of the fold on the chest, displacement of the implants toward the armpits, and distortion of the rib cage which can actually affect breathing in extreme cases.

I measure the dimensions of the patient chest breast and evaluate the amount and firmness of the current breast tissue and skin and choose a range of sizes and shapes that will adequately fill out the breast without overly stretching or distorting it. I then allow the patient to choose from that range of sizes and shapes based on her personal goals regarding size and cleavage.