Looking through the thread again, I don't find the most important distinction between the violas. The viola is the only instrument in the family which can be played in two legitime ways:
1. like the violin, rested between shoulder and chin: viola di braccio (= ital. arm)
2. rested on the thigh: viola di gamba (= ital. knee)
I don't know whether the different modes result in a different size,
I'm only a drummer.

Wilfried