The viola size is the first difference between violin and viola that you can notice easily, I mean if you have the two instruments in front of you to compare: the viola is bigger. The viola body length goes from 40 cm (15 ¾ in) to 42/43 cm (16.5/17 in) and it is larger accordingly, while the violin body is about 35 cm (13 ¾ in).
There are even bigger instruments, there used to be more in the 16th, 17th and early 18th century because viola music was very simple. Later they cut them down, to make them more comfortable to play.
Now it can be quite dangerous to play one of them, especially for a long time and for difficult pieces. It could cause physical damage to the player because he would tend to stretch his arms and left fingers and use a lot of pressure with the bow to draw a big tone from the instrument. Players of all instruments have already enough of these problems with normal sized instruments, we don’t need to make life even harder.
The famous viola player
Lionel Tertis
had a physical problem in his right arm and had to stop his professional activity. Maybe it was because of the big instrument he used to play, a 43.5 cm (17 1/8 in) viola made by Domenico Montagnana in 1717.
Some players, often violin players who occasionally play viola and find it too hard (and make me wonder "why do they still play it?"), use smaller violas (38 cm/15 in), but our
Tertis
would place these violas in the category of the "so-called violas". So, bigger is better... within limits.
Also viola strings are slightly longer and thicker and the viola is played with a slightly heavier bow.
This difference in size goes together with the different
viola tuning
and they give the viola its special, different timbre, its darker and warmer tone quality.
There is an easy way to choose the right viola size for you.
Stand with your left arm as you you were to play, put on your shoulder/collar bone the instrument you are trying. Then, with the help of your right hand to hold the viola, extend your left arm to reach the viola scroll. If you can barely touch the scroll with your fingertips, that viola is too big for you, it would be too much of a strain for you to play such an instrument.
On the other hand, if you can hold the scroll in your left hand, comfortably, without stretching, with your fingers gently curling around the scroll, that is the right instrument for you, I mean, that is the maximum size you should choose. If you are very tall with very long arms, you could also choose a smaller instrument for more comfort, not necessarily one has to play the biggest viola.
By the way, this is also the way to choose the right violin size for children when they start, either a full size or ¾, ½ etc.