My doctor wants me to have 350 cc, but I want 450 cc. Am I allowed to refuse his recommendation and get the size I want?

Thank you for the question.   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 which operation and/or 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 "small C cup" etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.  The use of  in bra sizers and computer imaging may be very helpful during this communication phase.
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 find that the use of temporary  intraoperative sizers ( with the patient in the upright position)  allows for accurate selection of breast implant size/profile to achieve patient's goals as precisely as possible.
I hope this, and the attached link, helps.

If you're not happy with your plastic surgeon and his recommendations, you should seek out a second opinion.

There are many factors that come into play when a plastic surgery chooses a breast implant size for their patient.  The chest wall diameter is one of the most important considerations, as well as the patients desired outcome.  It is a good idea to get several opinions on an implant size that best suits your anatomy.