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.

It is important to focus on a couple areas that are important to you and then have a conversation with your plastic surgeon about what can realistically be addressed.  For example, do you feel like you fill out a bra you like appropriately?  If you wear dresses that expose a part of your cleavage, does that area look good to you?  Identifying potential problem areas in this fashion allows you to ask what can be done.  Remember to write down your questions so that you don't forget to ask them when you are there at your appointment.