Hey minasan!
I saw lots of discussions regarding ito 糸 (string) sizes and equivalency in mm. I couldn’t find a pattern, BUT I found some good info by exploring Rakuten Japanese online shop. 
Link: Airtable
This compilation is according to the Sonica store in Rakuten.
- Size (standard + mm)
- Style (Jiuta not included, don’t know why)
- Material (Silk, Tetoron, Nylon)
- Brand
The sizes vary accordingly to brand.
This might help those looking for equivalent fishing line/guitar string sizes!
Feel free to ask for other sizes not included in this list, I’ll try my best to research them and update the list!
P.S.: Added the Kanjis of brands in case any of you want to research directly in Japanese online stores =)