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.