having worked for VS for a number of years, breast sizing is kinda in my wheelhouse as well as having a pair to call my own...

as stated above, breasts are measured around the ribcage/underneath the breasts themselves.  this determines "band" size for a bra.  measuring around the fullest part of the breast determines cup size.  for example:  if you measure as a 34" band, and the fullest part of the boobage measures 38", the difference is 4" which roughly equates to a "D" cup. 

in essence, for each inch larger of the fullest part to ribcage, you increase cup size.  1" more = A cup, 2" more = B cup, etc.

most women fall into the B or C cup range - 2 to 3 inches larger around full breast than ribcage measurements.

also, take into account the age of your model if you are doing nudes, as that will also determine shape as well as proportion outside of simple measurements.