Should I choose 375cc or 400cc? Concerned about them dropping and getting smaller when the swelling does down.

Thank you for your question. It is difficult to make a recommendation about size of implants without a proper consultation. Many board
certified plastic surgeons offer technology that can do 3D simulation
breast imaging so patients can view outcomes not only related to their
breasts, but to the entire body as well. I recommend discussing these
options with your board certified plastic surgeon, who can assist you further.

Thank you for the question.  Ultimately, careful selection of plastic surgeon and 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.
 
***Given your concerns, it is definitely worth spending more time (preferably prior to the date of surgery) with your plastic surgeon to discuss further.  One question that might be helpful to you: will you be more disappointed if you are slightly too small or slightly too big?  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.
 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 "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. 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 (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes for an outcome that you will be very pleased with.