What is the best way to figure out what breast size I should get during the DIEP stage II revision surgery?

The most common procedures I perform for a DIEP revision are touchups of the abdominal scar, removing excess tissue from the sides of the chest, minor adjustments to the position or size of the flaps for symmetry, or fat grafting to the upper part of the reconstructed breast.  You should think about what, if anything, bothers you about your reconstruction and talk with your doctor about what could be done to address it.

Typically the size is already established by the flap itself.  Fat is added through fat grafting.  Fat grafting
is a valuable tool in breast surgery.
This technique has gained more popularity over the past 7 years. There are many techniques used to harvest the
fat, process the fat and then re-inject the fat. Conventional suction lipectomy
is performed with a small diameter cannula, processed by separating the liquid
and fibrous tissue from the fat, and then placed into syringes for re-injection
or through a closed system.

I am confused?  Are you looking to add volume to you DIEP flap reconstruction?  You would have to be seen in person. Good luck.