Purchasing a String Instrument -Sizing and Arching's
There are two important considerations that the buyer must weigh with purchasing an instrument and those are the instrument's arching and size. We will discuss arching's first
Higher Vs Flatter
A top with a lower arching will usually help produce a stronger more powerful sound and a higher arching will generally create a softer warmer timber.
Correct measurements are very important. Sticking to standard measurements will help greatly, particularly for intonation, if you play on more than one violin. (for instance, you have one violin for solo work and another for teaching, playing outdoors, etc.) No set size goes with an age of a person. Not only does the length of the arm and fingers count but the strength is a factor as well.
For violas, the string length, like the body size, has never been standardized. There are some larger violas with a shorter string lengths, which might work well for somebody who has long arms and short fingers. Occasionally a larger nut can shorten the string length to accommodate those hands. I find that 15 1/2" up to 16" will generally work quite well for most viola players.