Cup Size After Mastectomy with Saline Implants?

You've have provided us with alot of information and I wish you the best of luck in your subsequent treatment.
Remember that the overall goal is to treat the primary and chemotherapy, if necessary, may delay or effect the reconstruction timeline.
Having said that, it has become increasingly difficult to pin down an exact size at this time.
Also the final size of the expander does not directly equate to the size of the implant. In fact, many surgeons "overexpand" for a prolonged period in order to create stretched skin that may mimic a real breast when a smaller implant is placed and has some "jiggle" room.
In addition, there are combination expander implants that act as the expander and have a separate port through which they are filled. This is commonly placed in the armpit area. When the right size or overexpansion has been achieved the fill tube is removed via a small 1 inch incision in one simple operation and the expander is left in place as the "permanent" implant. Now this is the ideal circumstance and frequently I find that I have to change the implant position/size/shape necessesitating a larger operation. However, I would discuss this option with your surgeon. It allows you to give your input as you proceed with the expansion process.
In regards to shape, there are round and "anatomic" also descriibed as anatomic expanders. Anatomic saline implants are available but to the best of my knowledge at the writing of this comment, anatomic silicone implants are not currently available.

The volume needed to achieve this result will differ amongst each woman.  You must take into account body habitus, height, weight, as well as the projection-type of implant used.  A smaller woman will require much less volume (cc's) than a much larger and taller woman would.  The chest girth in inches (36, 38, etc.) will not change as this is a measure of the chest circumference beneath the breasts.  The cup size (B, C, D, etc.) is measured by the projection from the chest.  Also, this is always variable, as not all bras are made standard and one brand may give a different cup size from another.  Determining this value during your expansion process, if you are having tissue expanders placed initially, may be the easiest way, as you could see the size as it expands and determine for yourself when you are happy with the size in clothing.  It sounds as if this is a larger volume, given your stated preoperative sup size.  However, since you will likely be going larger in size on the unaffected side, more volume must be placed to compensate for this.  It is difficult to answer your question without seeing you or knowing any additional information as that stated above, but certainly looking at your appearance during the expansion process and trying on different bras during may help.  

A tissue expander is thicker than a regular implant due to the medal valve and thicker shell. The tissue expander alone carries approximately 75 to 90 cc in volume. With 450 to 500cc saline injected into a tissue expander, you are looking at the final volume of 600cc implant. Without any breast tissue, a 600cc implant would give you a C to D cup depending on your height and chest circumference. I recommend that you seek consultation with board certified plastic surgeon who specializes in breast reconstructions.

Breast size after reconstruction is a challenge.  I currently use the Jockey Bra system for this.  I have found this system the most effective in establishing a bra size.  Each 200cc or grams roughly represents a cup size.  Please find an experienced Board Certified Plastic Surgeon
and member of the Aesthetic Society using the Smart Beauty Guide. These Plastic Surgeons can guide you on all
aspects of facial surgery, breast augmentation and body procedures including
tummy tucks or mommy makeovers!

There is no way to guarantee a particular bra size. Bra sizing varies greatly between bra manufacturers and a C in one bra will be a D in another. What matters more than the assigned bra size is the way the implant looks on you. The best option for your body and aesthetic goals can be determined in a thorough implant sizing session.Implant sizing depends on several factors. One of the most important factors is your breast width. Generally, your surgeon will measure your breast width, and then provide you with a range of implant sizes appropriate for your native breast size. There are more nuances to it than just what I've described, but this approach works for most women.I usually have my patients bring in a large bra and a tight t-shirt to do sizing. I'll then choose 3-4 implants that I feel are appropriate, and have my patients place them in the bra under the tight t-shirt. My patients can then look in the mirror and get a good sense of what they will look like with the provided implant sizes. My patients like this approach and get a great idea of how they will look.By using this technique, your surgeon can outline a range of appropriate implant sizes that will be aesthetically pleasing, and you make the final decision.I hope this helps. Good luck!