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

Unfortunately, words alone cannot allow us to arrive at any conclusions or suggestions. Without a physical examination or at the very least, photographs, it is difficult to respond with any specificity or accuracy, but the best way to determine which breast implant size is best for you is by first finding out your specific wishes not only with words or letters as in cup size or “not too big” or “natural”, but also with a VISUAL understanding of what you hope to look like. This can be accomplished either with photographs of patients whose breasts and body type look like you, or ideally with photos of patients not only whose breasts and body type visually are similar to yours, but are also a similar height, weight, and pre-operative breast size. In our own practice we have that unique functionality on our website.
Unfortunately the most common reason for a woman to be unhappy after her augmentation is feeling as though she is either too big or not big enough. I do not place the onus of selecting the breast implant size on my patients since they are coming to me for my advice and expertise, but I do ask my patients to be as specific as possible about their aesthetic goals and I then advise them accordingly. The final decision as to which implant will be used in surgery is made in the operating room after inserting "sizers" that show us exactly what you would look like with a 405 versus a 450.
I would advise you to discuss your concerns once again with your plastic surgeon, use photographs to communicate your desired end point, and make sure that your own sense of aesthetics aligns with that of your plastic surgeon. Your idea of “natural” or “pretty” or “proportionate” might be different from your plastic surgeon’s and it is very important for you to feel confident that you are both of like minds as far as the goals. Best of luck to you and I hope you are successful in achieving the enhancement you are seeking.

Thank you for your question. Implant size depending on your breast diameter, your goal and anatomy. I recommend you talk to your PS and try the sizers again and decide.