International Chart of Finger Ring Sizes
This free PDF file is a handy chart of finger size diameters and circumferences. It is in both MM and Inches and 1/4 size increments. It has comparison sizes for several international size systems. finger sizer ring sizer
These numbers should be correct for a finished ring size. Since each 3d printing or milling system and subsequent downstream manufacturing will have unique shrinkage or distortion, these numbers may need compensation scaling in your modeling process to arrive at a correctly sized finished product.