@pallas Hm, not sure. I usually just use old lines off my boat and either make it until I run out of line, or cut off the excess. Not sure how much line is needed to make just the central part of the mat, say.
I think this one was about 25 m long x10 mm thick(for a medium-sized mat)?
I've made larger mats with 37 m of rope(12 mm thickness).
You won't need 25 m if you don't make the 4-5 coils around the main mat.