This is a very common question. The answer, it depends...

The answer to this question is that it depends on numerous factors.  In you particular case, however, it is much easier to get a feel for your desired implant size because you have had a double mastectomy.  Therefore, with the breast tissue being gone, the implant is going to make up the new breast.  external sizers in the office should give you an excellent feel for the "right" size implant.  In addition, you will probably undergo tissue expansion which will be another opportunity to assess the "proper" volume implant.  The process of expansion can be trying, but think of it as an opportunity to try out lots of different sizes of implants along the way.  Best of luck!