FORUM

Prior to the 1970s/80s, before blind auditions became the norm in professional orchestras, there was a lot of discrimination against people who played smaller violas or violas in non-standard shapes.

Today, professional orchestra auditions take place behind a screen, so the audition panel makes the decision without ever seeing the candidate or the instrument being played. All that matters is how it sounds. We're now seeing a lot more violas under 16" being played in professional orchestras than before. That said, violas smaller than 15-1/2" are still rare at professional level. Even in blind auditions, the smaller a viola gets, the more likely the player is to be at a disadvantage simply because of the acoustic limitations.

In community orchestras, even elite/semi-pro ones, small violas are not nearly as much of an issue as long as the viola is good enough to be responsive throughout its range. There's a shortage of amateur violists in most places, so even if the orchestra has high standards, they're unlikely to turn away someone who has the technical chops to play at that level. My semi-pro orchestra even had someone playing a 14" viola for a couple years, and someone else played in the viola section for over a decade with a 14-3/8" viola. 

Just to be clear, there's a fair number of pros playing 15-1/2" violas. 15-1/2" is the smallest size that's common in professional orchestras. It's smaller than 15-1/2" where the numbers seem to drop off rapidly. You can occasionally find violas as small as 15" in professional orchestras, and there are more of them now than in past eras, but I don't know of any smaller than that.