Hello ,
I am currently (very slowly building a shamisen well …)
I think the measurements is dou : 20 cm x 20 cm and 1 cm thick.
I believe (of my head ) sao is 98cm long including the neo thing and the headstock (i tend to build bass guitars so um i dont know what they are called) and about 6-7 wide at the fingerboard I know its measurements are wrong now but I started before I discovered kyles shamisen of Japan or this site
Hello Dafydd,
For a tsugaru dou, it’s about 22cm wide by 24cm long. The fingerboard is actual 3cm wide.
I think you will find Shamisen of Japan very helpful for building your shamisen. All of the dimensions, templates and instructions are presented there. 
Cheers!
Kyle
Ah thank you kyle
I will make changes to the fingerboard but I can’t bring myself to restart dou as it’s a really nice bit of maple but i
Be a bit out with my guess
No worries Dafydd! I just measured the dou of my very first homemade shamisen, and it is actually 20cm by 20cm as well! It sounds fine too. 
(And made from maple too)
The size being different from standard doesn’t actually matter. It’s homemade, so you can do whatever you want with it. If you like the sound, that is all that matters.
Oh that’s reassuring to hear
And I just measured my neck it’s 3.1 cm thick so it’s about right I ment 6-7 in the headstock and if your interested the Sao and fingerboard is made from a single bit of cherry (I tried posting pics but it would not behave on my phone so =( )