Andrew Fryer said

 

I know two violists. One plays a 15" viola made in the 18th century.

The other used to be a pro until she got a neck injury, so now she's an amateur. Hers is 16 3/4" (42.5 cm) and was made in the 20th century by a luthier in the North of England out of wood salvaged from some broken cellos!

If I ever wanted to buy a viola, which is very possible, I'd just go for a 15" (I'm only 5'5"). I see that Stentor, for example, lists that as a standard size. No, if you go for a basic Conservatoire model, they do it in 14"!  

I know a whole bunch of violists from the various ensembles I've played in recently, with viola sizes ranging from 14-1/2" to 17". At one point, maybe 8-9 years ago, I played in a community orchestra with someone who stood 6'7" and played a monster 19" viola! In my current semi-pro orchestra, there are two violists who play a 14-1/2" and a 15" respectively, and the other seven are all between 15-1/2" and 16-3/4".

14" is exactly the same size as a full-size violin, of course.

My own instrument is a 15-3/4" Tertis pattern viola, made in Phoenix, Arizona in 1979 by a German luthier who moved to the US in the early 1970s. I'm 5'7", and my arms are long enough to handle a slightly larger viola, but I have short fingers so I can't play an octave double-stop on anything larger than what I have.

Irv said
Hi Andrew.  I have a couple of 14” violas.  Their ribs are quite a bit higher than a violin. All other dimensions are the same.  If I had younger children and needed a fractional sized violin, I would strongly consider getting the similar in a viola and string it as a violin, for fractional violins are notoriously bad sounding.

If you want to play a longer viola, check out the violin metrics thread on this forum and see what the Fiddlerman Shop did for bocaholly (lengthened nut to convert 4/4 violin to 7/8 violin).  I saw real potential in that modification.  

For the price range I'd be shopping in, I wouldn't want to make any major modifications; the risk of significantly reducing the instrument's value is too high. (Also, I'm about 95% certain I've already purchased my last string instrument, because I'd never need to upgrade even to play in professional ensembles.)

Also, with the viola, lengthening the nut comes at a price: it becomes harder to play in high positions because the body starts to obstruct your hand sooner. I routinely play orchestral viola parts going to 9th or higher positions, which is inherently awkward on any viola. I really don't want it to get any harder. Even on a violin, that modification might not be desirable for someone who plays Romantic and post-Romantic solo rep and 1st violin parts.