The length of the back is the measurement from the end of your neck (from the bone) to ... wherever you want! Let me explain: the length of the back determines the length of your coat, so it's up to you to determine how long you want your coat to be.
To get the width of your shoulders, you have to start from the acromion (the pointed bone that marks the intersection between your shoulder and your arm) on one side and go to the acromion on the other shoulder, following the back.
Your sleeve length is the measurement from the shoulder bone to the wrist. The tape measure does not follow the arm. (Add a centimeter to the measurement for outerwear pieces so as not to be too tight.)
Also known as the waist circumference, this measurement is the horizontal circumference of the body measured at the deepest point of your abdomen, which is slightly above or below your belly button.
DETAILS THAT SHOW YOU THAT A COAT IS THE RIGHT SIZE (AND OTHER TIPS)
When you try on a coat in store, pay attention to the following points to make sure it fits like a glove:
The shoulder is clean, without excess material,
Try it on with a sweater or a jacket to have some room in winter,
you can close all buttons,
You should feel held but not compressed,
The sleeves slightly cover your wrists,
For the length: prefer it above the knees for the coats, and just under the buttocks for the cabans (don't hesitate to shorten them if necessary).