ViolasViola players are difficult to please - no two players ever seem to want the same length instrument, or if they do, they want different widths and rib depths and so on.
A list is therefore meaningless - it cannot possibly convey enough information to help a potential player.
However my stock is changing all the time, of course. It is best to phone and let me know the kind of thing you are looking for. I'll be able to tell you if I have anything suitable.
I have several 19th Century German and French violas, including a vast German instrument, 16 9/16 in and wide with it. It's nothing special, just a (good) factory thing, circa 1880 - but in clean condition and somehow attractive. It's for those who like a cello under their chin, really.
All else being equal, small violas are, actually, inherently louder than large ones.
Small violins, like those by Guarneri, are even more powerful than those of Stradivari. A well set-up violin is every bit as loud as a cello, and quite a bit louder than a double bass.
Acoustic volume is not dependent on spatial volume. This still holds true for the lowest string; the C string. Of course a small viola won't sound so much like a cello - but it'll still sound different from a violin and be able to play the viola repertoire.
If you are uncomfortable with a standard size (say, 16 inches) instrument then do consider something smaller . . . such as my superb viola by Bernhard Simon Fendt. It's just over 15 inches, but is, perhaps, rather too powerful for the average string quartet. Images here