The mini is recommended if the back ear distance is less than 23.5cm.
You can put a string at the back of the head from one ear to the other and then measure the distance. Shokz has a size guide on their product page:
https://cdn.shopify.com/s...

This covers all head sizes, because it is a bone conductor headphone, and the headband is very flexible, i.e. it rests next to the ears on the bones. It's not worn directly over the ears.