@Leaviathan 

The violin strings are closer together. The smaller the violin, the closer the strings. 

I have a 15”, 15 1/2” and 16” viola. The smaller the viola, the closer the strings.

When I googled, I found a luthier’s chart, it showed narrower distance between strings, also.

@Leaviathan 

Here is a chart with viola specs:

Alan Goldblatt, Luthier Viola Spec Chart

Maybe you can find what you need here? 

I know I had a cello that recently traded in that had a narrower than normal fingerboard. The strings were a little closer. The rest of the cello normal 4/4 cello. I traded it because when shifting, the C and A strings were too close to the fingerboard edge. It made it hard to learn shifting because it felt like my fingers were falling off the edge. There are a few made like this for smaller hands. Could be for other reasons, too, but that was the explanation at the violin shop. It was comfortable until I started shifting.

Given that, I would think there could be the occasional viola, or any string instrument, with slight variations. 

Are you making a viola?

@Leaviathan  

My height is about 5’6”. I can’t remember my arm length. I know that bending the wrist to do the strings was a bit much. I got sore and tired more easily. It tightened my arm too much because the viola was actually a little too long and stretched my arm too much. It didn’t look like it, but when playing it, I could feel it. I would think that if you have tight arms and are not extremely flexible, I would go with 16”. I would think it would be much easier to finger. The change in space between strings is minimal. I was concerned with that because I have fat fingers. Fatter than the long slender fingers all those people in those violin and viola instruction videos have. So annoying. Normal sized people need to make those videos!

That little 1/2” made a world of difference when I downsized. The sound is just as full as the 16”. I actually tried the same model in the 15 1/2” and 16” and they sounded the same. Can’t remember which one it was. It was not the 15 1/2” one that I bought. The pawnshop rescue 15” is a little hollower sounding (not as robust), but still a nice tone. That would be too small, anyway, but just a reference as to sound difference. Between the 15 1/2” and 16”, there was none.

My 15 1/2” viola is a Rudoulf Doetsch, as is my violin. They are very warm with a beautiful sound. They are keepers. I have am waiting for my Fiddlerman Concert Deluxe Violin because it is brighter and the happier songs a d fiddle music will sound better. That is how warm they sound to me. Fiddle music sounds sad on my violin because of the warmth. It is lovely for all my other music and I am not complaining. It is exactly the sound I wanted. Warm violas are so much more beautiful than bright violas. In the Viola section (I think) I posted a link to a Ted Talks for a lady who is talking about the viola. It is so entertaining and so in spot! She also played the viola. Exquisite. I will see if I can find it for you. 

Leaviathan, where do you live? If the shipping will not be too much, I could send you my non-working 16” viola and you could see if you can do that. I would not want it back. If you like it and think it sound good, maybe it would be worth getting a new fingerboard, but I doubt it. It was only a $67 test viola to see if I liked it. It depends on how much it would cost me to ship it because the viola is really not worth much. You can PM me if you don’t want to post location publicly.

Argh! I have to get a new shoulder rest for my viola. The one for my 16” that I traded doesn’t fit, even with the legs in all the way. I keep forgetting until I pick it up to play it.