I'm 5'4", 142lbs and aiming for a full C cup. Should I choose 405cc or 450cc?

The best cosmetic result in any particular breast
augmentation patient depends on a variety of factors, including: Your
individual anatomy, desired outcome, realistic expectations, a thorough
discussion with the plastic surgeon about the options, and an understanding of
the pros and cons of any particular implant choice. Proper sizing is not just
about the number of cc’s. or the type of implant. The thickness of your tissue,
breast dimensions which include the width, height, and projection, as well as
chest wall width all need to be considered when choosing an implant. Trying on
implant “sizers” of various shapes and volumes while wearing a tight t-shirt,
bra, or bathing suit at a preoperative visit will help you and your surgeon
choose the optimal implant.

Keep in mind that following the advice from a surgeon on
this or any other website who proposes to tell you what to do without examining
you, physically feeling the tissue, assessing your desired outcome, taking a
full medical history, and discussing the pros and cons of each operative
procedure would not be in your best interest. I would suggest you find a
plastic surgeon certified by the American Board of Plastic Surgery and ideally
a member of the American Society for Aesthetic Plastic Surgery (ASAPS) that you
trust and are comfortable with. You should discuss your concerns with that
surgeon in person.

Robert Singer, MD FACS

La Jolla, California
 

My best advice: more communication.
It is definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
One question that might be helpful for you to consider: would you be more disappointed if you are slightly too big or slightly too small after the breast augmentation procedure?
Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
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 are 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. For example, I have found that the use of words such as “natural” or "full C 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. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
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, after the use of temporary intraoperative sizers.
I hope this helps. Best wishes for an outcome that you will be very pleased with.