NOTE: It is best to have someone help you measure.
START AT THE BACK OF THE NECK.
Place the tape measure at the center of the back of the neck. You do not need to be wearing a stole, but it would be a good idea to wear your robe if you have one.
DETERMINE THE LENGTH.
Drape the tape measure down the front of the robe. Look in a mirror to determine your desired length.
HAVE ANOTHER PERSON LOOK AT THE MEASURING TAPE.
Stole length is usually a matter of personal preference, typically somewhere between the knee and a few inches above the robe. Clergy who use a cincture may desire a longer stole. Traveling length stoles usually are about waist length.
Most of my stoles have a maximum length of 54 inches.