So if the Hosozao is the smallest of the shamisens? and from what little information I have found is that the sao is the same length, but the dou is smaller? anyone have measurements of the dou?
my dou is 12 cm deep (13 if you count the nipples), 21 cm across, 21 cm in length
so the dou is a square box 21 cm by 21 cm?
Hosozao shamisen are technically the smallest, though the size does vary within that variety of shamisen. I have several hosozao shamisen in my closet, and some of the dou are 20cm long, others are 19cm long.
It’s nice, as the variation shows nothing’s set in stone, it’s a matter of preference. 
thanks Kyle