Approximate sizing and measurements. Exact size/dimensions will vary from maker to maker. Note that 1 inch = 2.54 cm
If you have access to an instrument, ask the student to hold the instrument as if they are playing. The student should be able to cup the majority of the fingers around to the top of the scroll.
Always choose the smaller size if the student is in-between sizes, because a larger instrument will hinder the student’s ability to play properly.