The best way to find out what size violin you need is always to ask your teacher or another violin professional.
If you for some reason have to decide what size you need on your own, here are some helpful hints:
HOW TO MEASURE WITH AN INSTRUMENT
The student should hold the violin in playing position, supporting the instrument with their chin on the chinrest. Then reach out the left hand and grasp the scroll. If the palm and fingers of the left hand are able to comfortable cup the scroll of the violin, without stretching and while bending the elbow slightly, you have found an instrument of the right size. Make sure the violin is fairly close to the neck and is lined up on top of the shoulder rather than facing in the front.
HOW TO MEASURE WITHOUT AN INSTRUMENT
Persons under five feet in height usually need a fractional size, and over five feet a full (4/4) size. For a child, ask their teacher what size they prefer, as this is also a matter of taste. The following chart will give you an idea: using a yardstick, measure between the neck and the center of the palm. When in doubt, always choose the smaller size.
Let the student extend their left hand fully. Place a yardstick or measurement tape on the shoulder against their neck – just where the edge of the violin hits the body when you play – and measure from the neck to the middle of the palm.