I Am 5 Ft 4 in Tall 115 Lbs. Are 450 Cc Implants a Good Size for Me ?

Sizing is an art and not a science.  There is virtually no difference between 425 and 450 cc implants.  The question is what size you are looking to be after the surgery and, unfortunately, you haven't given any indication of what that might be in your question.  Either implant will produce a C-D cup breast, approximately based on your body size.  Given that you are thin, the implants should be placed under the muscle.

Thank you for the question.

Unfortunately, it is not possible to give you precise advice based on the information you have provided.

Communication with your plastic surgeon will be key to achieving the results you are looking for.  

As you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.

In my practice I use goal pictures to communicate with patients.  With this technique patients are able to demonstrate what they are trying to achieve and what type of looks they do not like as well. 
I also use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible. 


By the way, the most common regret after this operation, is “I wish I was bigger”.

I hope this helps.