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I too read that criterion about the scroll for the size. The luthier actually verified that before I bought my viola n°2.
I just went to quickly check on my viola, and yes, I can grab the scroll comfortably, without being extra-stretched at the elbow (I can even curve my fingers all around it). So yes, I suppose you should go for a smaller one.
My viola is 41 cm, so that would be... converting... 16''.
A smaller viola should still have a "viola sound", or there's no point playing viola, right ? Best to try extensively.
I read (in a book by Paul Rolland about strings playing) that viola position can differ slightly from violin position, too. Like, you can hold it a bit more to the front and have the scroll dip a bit more towards the floor. Important point is being comfortable.
E.g. look at her :
(I want to play that someday, too. Beautiful.)
The wrist is not as tensed up and my arm feels much more relaxed. Physically wise, I think it will be safer.
Agreed (to clarify, I wasn't saying you should stick with 16'' instrument). It's worth it going towards smaller sized ones. Do the scroll test with a 15'' or 15''1/2 when you find one...
Good luck in your search for the right instrument, I'm sure it exists :). I know someone (adult) who switched to a 3/4 sized violin for the better (after years on 4/4, because "as an adult you must play on a 4/4 violin" that's what teachers or sellers told her), she's just not tall enough for the full size. You can't get a good sound if you're all tense - no matter how good the instrument. Plus, one can do without the pains
Keep us updated !
There are a lot of factors that go into viola sizing and technique. Arm length is one of them, but also consider hand size. If you have short fingers, you may be more comfortable with a smaller viola even if your arm can handle something larger. Also, violists with shorter arms or small hands may make other technical adjustments such as holding the left thumb opposite the 2nd finger (instead of 1st), or playing with a more rounded left wrist. Violists tend to hold their instruments more forward than violinists, as @wtw pointed out already.
Violas vary a lot more than violins, because the size and shape of the viola is a compromise to begin with. I understand that the Stradivari and Guarneri pattern violas are closer to the violin shape, while the Maggini and Tertis patterns are wider and taller, so as to have more volume for their length. As far as I can tell, the difference between Maggini and Tertis pattern violas is that Maggini violas get their extra height from a higher arching top plate, while Tertis violas get their extra height from taller ribs. (I play a 15.75" Tertis pattern viola.) Recent decades have seen more unusual variations too, e.g. the Iizuka pattern with a cutaway upper bout, and the asymmetric Rivinus pattern that places the whole body of the instrument at an angle.
Because of the variations in shape, it's really difficult to have a side-by-side comparison of different viola sizes. Generally, for the same shape, larger violas will have a darker, richer sound, but small violas with a dark sound can be found. What you find more often with small violas is that there is a compromise between projection and richness of tone. It seems to me that smaller violas can either project like larger violas, or have a dark sound like larger violas, but rarely both. Also, the choice of strings may differ. I've noticed in my orchestras that the people who play smaller violas tend to use brighter and more direct-sounding strings in order to get more sound out of their instruments (and I do as well), but it is at the expense of tone color.
I've noticed that younger violists tend to play the largest viola they can handle, while older violists (including pros) often downsize for comfort. There are plenty of excellent 15" and 15.5" violas on the market; they are both considered fairly standard sizes.
There really isn't any need to buy a different string length for a 15" viola; for almost all brands the same string is used on 15" and 16" violas. The most common string lengths are typically labeled as being for either 15-17 inches or 15-16.5 inches. You only have to get an under-length string if your viola is smaller than 15".