Breast Implant Size for 34C After Mastectomy?

You best bet is to use the tissue expander to guide you in your decision of breast implant size.
The average A cup requires approximately 300-375cc to become a C cup.
Try on different bras with your current tissue expander that would achieve a C cup.
Use this technique to guide you in your decision of breast implant size.

Your surgeon chose your expander size based on your mastectomy volume and desired breast size at the end of the reconstruction. Once the expansion is complete, your surgeon will choose your implant based on the final fill volume and your expectation

Cup size is a function of what's present and what's implanted. Since you have had bilateral mastectomies, your eventual cup size must be provided by the implant used. My breast cancer reconstruction patients always indicate their desired cup size however we discuss that this result is contingent on the availability of adequate (mastectomy) skin coverage. The other factors which impact the creation of a soft, mobile breast are: a) the presence/absence of radiation therapy; b) the intrinsic width of your chest, which is a guide to the diameter of the implant which can be used. However the diameter of the implant varies with the style of the implant, with the "high profile" implants, which project more, being narrower. My breast cancer patients usually prefer the "moderate plus" or "high profile" styles because we can take advantage of the implant measurements to meet the patient's expectations.
The current tissue expanders also have a certain diameter, which your surgeon selected, preop, keeping your objectives in mind. At your second stage reconstruction, in which the tissue expanders are exchanged for permanent implants, you should participate in the selection of implant. Good luck.

The simplest way to determine the size of the final implant with expander reconstruction is to note the amount of saline in the TE at the point it reaches your desired size during the course of expansion. That can then determine the size of saline or silicone implant to be used. I prefer to overexpand a bit before the exchange.