Violin teachers? Need advice from those with knowledge of choosing an appropriate violin size for a student.?

I am a music teacher. Piano, keyboard, recorder are my instruments I am experienced in teaching.

Violin is something that have taken up in the last decade. I would be able to teach it to beginners. I realise that if your a child you would need a 3/4 or less but how do you know exactly the right size?
There is 1/2, 1/4, 3/4, and 4/4 violin (and more I think).

I have heard that there is a method of measuring the arm of the child to choose the exact size that is right for them. How is this done and what are the correct measurements to know this? I can’t find it anywhere on the net.