How do I figure out what size mount will fit my stamp?

Just measure your stamp’s width and height (mounts are measured in millimeters).  Add 5 millimeters to each measurement, then refer to Mystic’s mount list to find the corresponding size.  Or simply go to Mystic’s website and search by the stamp’s description or Scott number.  Mount sizes are listed on each stamp’s page in the lower right corner of the stamp image.