Hello there, welcome.
Without being there in person to I.D., I’m going to depend on you to measure and tell me the data points.
1 -> Lay the shamisen on the ground, and measure from the tip of the tenjin( 天神) to the end of the nakagisaki (中木先). Do not account for the tenjin’s curve. A tanzao (単棹) will be 90 - 95 cm long. A seisun (正寸) will be 98 - 101 cm.
2 -> Measure the neck’s width at just below the chibukuro (乳袋). Neck types exist on spectrum of thin (hosozao - 細棹) to thick (futozao - 太棹). Typically, hosozao are 2.6 or less. Typically, futozao are 2.8 cm or more. The range in between, including the limits (2.6 ~ 2.8) are generally termed chuuzao (中棹).
Moreover: Modern futozaos tend to be 3.0cm or larger, whereas antique hosozaos were perhaps closer to 2.3. You may also find the neck taper which some instruments exhibit to be a complicating factor.
3 - > Remove the doukake (胴掛). Measure the length and width of the dou (胴). Do not account for the rounded sides. Measure the flat top. Then, we can compare it to the following chart:
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)
By looking at the dimensions of the neck and body, we may be able to identify the shamisen’s intended subtype.
Please note that ancillary features (azuma/en sawari; itomaki size; hatomune) are not perfect identifiers, especially when dealing with older instruments. However, we may end up referring to them to further clarify things.