If by ”rudder pin” you mean the pivot bolt, mine (original) is 40 x 5 mm, excluding the screw head. This is just right only if you use the original mushroom-shaped plastic spacers. Most people today use a 9 mm bolt instead. The length of that would be the same as that of the ”thin” bolt + a few mm more.
Ha Had some semantic problems there... but the answer to your question is, it doesn’t really matter. Any piece of cheap nylon or polyester line about 25 cm long and some 3 mm thick will do. You can use elastic here, too. I might post some suggestions tomorrow.
Not many use the pin, though. The rudder downhaul line is what in fact keeps the tiller in place anyway.