Violas must be fitted to each player. The size makes a difference in both the distance the musician must reach to the fingerboard of the instrument and the distance the notes are separated on the fingerboard.
So how do you know what size is right?
Usually, the string teacher measures the student for the appropriate size. If your teacher recommends a size, you should get that size regardless of any measurements anyone else might do.
If the teacher hasn't made a recommendation yet, you can simply place the viola under the student’s chin in playing position. If it's the right size, the student should be able to cup the scroll (the far end) of the viola with their hand. If the instrument isn't available, you can also use a "fitting yardstick" marked with the instrument sizes. When the end of the yardstick is placed against the student’s neck, it will stretch the length of his or her outstretched arm. The correct size is the one that is closest to the student’s palm.