What Macom bra size should I get?

Thank you for the question. It will be important to speak to your surgeon in regards to bra sizing for after surgery. In my practice, the office provides the bra for the patient so this is not something my patients need to do prior to surgery.
There is no "standard" when it comes to the use of bras and what type of bras to use after breast surgery.  Ultimately, careful selection of plastic surgeon will be the most important decision you make; and then follow his/her recommendations.
  I am an advocate for the use of supportive bras after breast augmentation surgery.   In my opinion, the use of the supportive bra may help prevent breast implants from moving into the "paths of least resistance": down and/or out.  In my practice, patients wake up with a surgical bra in place. This surgical bra, although not attractive, is supportive and used for about one month after surgery (some of my patients have been known to burn the bra ASAP). Then, patients transition to a soft sports bra. Wired bras are used when breast implants have reached their “final position” and patients have full sensation along the breast skin. Generally, the longer I have been in practice, the more I appreciate the use of supportive bras for the first several months after breast augmentation and revisionary breast surgery. 
Best wishes  in your decision-making process and foreign outcome that you will be very pleased with..

Every surgeon has his/her preference about surgical bras and how they should fit. I would suggest calling your surgeon's office and ask to speak to his nurse about your size.