What size breast implant will give me a small D cup after a double mastectomy?

This really depends on the mastectomy flaps.  Putting too much fluid in an expander can compromise the blood supply to the mastectomy flaps and cause necrosis of the flaps.  Most people generally start with 50-100 cc.  I believe it is wise to stay on the conservative side as flap...

Its really hard to give you specific advise without pictures or an exam. In general, a big change in breast reconstruction is getting enough profile in the implant to avoid the "flat" look you are describing. In my practice I tend to use ultra high round implants or shaped high profile implants....

After mastectomy there is less skin. To remove more in any pattern is not wise unless it is replaced with a lat doors flap. This is a good option and adds better tissue to cover an implant.The nipple and scar will likely be improved. In experienced hands this can give an excellent long term result.