How do doctors decide what size implant to use during breast augmentation surgery?

Thank you for the question. There is no accurate correlation between size of breast implant and cup size achieved.
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you're looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural” or "C or D cup”  etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
 The use of computer imaging technology may also be very helpful during this communication process.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this (and the attached link) helps.

Every plastic surgeon needs to make that decision for themselves and their patients based in their judgement and experience.  I recommend size based on my experience using implant sizes in the past considering each patients desires and goals as well as their individual build.  I almost always commit to a size the morning of surgery and don't stray from the plan I tell the patient.  That way there are no surprises or disagreement afterwards about me "changing plans" in the middle of surgery without the patient's knowledge.  But each surgeon makes that decision on their own.  You will probably get different answers from each doctor.

Sizing should be based on tissue characteristics, just like when you buy a shoe the shoe has to fit your foot, if your surgeon does enough measurements they should be able to give you a range of sizes that will fit your body