Based on what we've found on "modern" instruments, your instrument is whatever you want it to be. If you put fiddle strings on and tune it GDAE it's a fiddle. If you put viola strings on and tune CGDA it's a viola.

There are almost certainly some "conventions" that say that a viola should be a little "thicker" and probably a little wider, especially at the lower bout, than a fiddle of the same string length, but I've been unable to find any consistent modern dimensions.

Antonio Stradivari: His Life and Work (1644-1737) by Hill, Hill, & Hill, Dover reprint 1963, gives "basic" dimensions for about 50 Stradivarius and contemporary era violins. Stradivari apparently made only about 10 violas, and those made by the Amati and Stainer are similarly few. Dimensions for four "large violas" and six "small violas" are given by the Hills, but there seems to be little "trend" to the dimensions, and the difference between violins by different makers – or even those made by the same person at different times – seem to be at least as large as the differences between violins and violas of any given maker.

The "large" violas of the 1600-1700 era all have "boxes" of 17.5 to nearly 19 inches in length, so would be easy to recognize as "not violins." The more commonly made "small" viola had "box lengths" of about 16.25 to 16.87 inches, somewhat longer than the 13 to 14.8 inch length for boxes of violins by the noted makers of the same era, but not too significantly different from many violins made by other known makers of the same time period.

Widths across the upper and lower bouts ranged from 6.2 to 8.4+ for violins of that era, while the small viola boxes were in the range of 7.3 to 9.7 inches. Generally, the lower bout is wider, but there are several examples of distinguised "upside-down" violins where the upper bout is significantly wider than the lower. Even comparing a given violin to a viola made by a same maker within a short time span, there is no consistent difference that I can discern by "optical filtering" of the information available. It is safe to say only that the viola box would probably be "bigger."

The most consistent difference between these early violins and violas appears to be in the box thickness, with the violins from about 1.12 to 1.8 inches, and the small violas 1.2 to 1.6. The ranges obviously overlap, but for a given maker at a given time there appears to have been a tendency for the viola box to be about .12 thicker than the "matching" violin.

It appears that these early makers rarely made a viola except when a wealthy patron wanted "enough instruments for a quartet" and didn't want two "identical" instruments in the set. The esteem (lack of) in which the viola was held at the time perhaps accounts for a couple of the violas being so ornately decorated (inlaid) as to be all but "unplayable," the assumption being "well nobody will ever play them anyway, so it's a chance to show off with decoration."

If it looks like a slightly "chunky" violin, it might look like a nice viola. Otherwise, string it to do what you want to use it for, and enjoy. (Tell folk it's a rare "Stradamati.")

John