Ooh, interesting.
If you measured accurately, my money’s on a shamisen built with kouta in mind!
For reference, here’s what my notes list:
Associated (non-exhaustive) genres are in bold. When I lack direct experience with or sources for an instrument’s dimensions, I produced extrapolated values. These are in italics.
Regular text indicates dou size and the “official” dimensions in centimeters, as converted from Sun-Bu-Rin.
Nagauta
Nagauta - 19.7 x 17.88
Min’yo / Hauta / Kouta
Go Rin Dai - 19.85 x 18.03
Jiuta
Ichi Bu Go Rin Dai - 20.15 x 18.33
Ni Bu Dai - 20.3 x ? (18.48)
Gidayu / Tsugaru
San Bu Dai - 20.61 x ? (18.79)
San Bu Yon Rin Dai - 20.73 x ? (18.91)
Yon Bu Dai - 20.91 x ? (19.09)
Tsugaru
Go Bu Dai - 21.21 x 19.39
Roku Bu Dai - 21.52 x ? (19.7)
Nana Bu Dai - 21.82 x ? (20)