Door sizes are typically based on the hinges you plan to use. Those hinges will have a specific overlay, which is essentially the amount the door overlaps the face of the cabinet case or face frame.
If you plan to reuse your existing hinges because they are fairly new, you may be able to use the existing door sizes to order your new doors. That said, we typically see customers replace the hinges as part of the reface. When that happens, the new door sizes need to be based on the cabinet opening dimensions plus the hinge overlay. Historically, a 1/2″ overlay is the most common.
https://www.barkerdoor.com/Standard-1-2-overlay-hinges-s/104.htm
If you want to do a full overlay reface, take a look at the link below and watch the video. It is a little more complicated because we vary the overlays depending on the location of each opening, but the end result is a cleaner, more modern look for a cabinet reface.
https://www.barkerdoor.com/Full-Overlay-Tutorial-s/188.htm