Unlike other string instruments, there is no standard size for violas. Is there a particular reason why for violas?
There are standard sizes for Violas, mostly in the modern, mass-manufactured market. In the U.S. they are sized in inches, and have half-inch sizes available, starting at around 11 inches and going up to 17 inches. The size choice is based on the player's arm length, and you generally go for the largest size that is comfortable to play.
You will see more variation in handmade Violas than violins however. Like the violins, there are certain antique models that current models are fashioned after. For example, you often see violins modeled after certain Stradivarius' instruments. The classic makers experimented somewhat, and there is some variation in the instruments even with a single maker over the course of their making instruments.
With the violas you see a wider variation among the historical makers' models in size and dimensions, so modern patterns based on them will also have more variation. There also seem to be more makers that are copied than the violins, thus more sizes and shapes.
The size of the Viola is a trade-off also, making them smaller than they should be for optimal tone in order to increase play-ability. Some dimensional changes are made to to the sizes to help get better tone in the smaller size.