I would completely ignore what came in the kit. I'm sure no thought was given to scale. They probably put the same "thread" in the 1/60 and 1/220 kits too.
0.5mm sounds about right, as that'd be 2" at full size. And down from there.
The anchor cable should be thick say 2mm and find some nice string with a twist.
To be honest, I usually go with what looks right. As Don says, vary color and size and it'll look good.
Also recommend running it through beeswax. If you buy your thread from Model Expo or someone like that, Blue Jacket etc., they'll have it too.
Careful though, grown men have disappeared into those sites never to be heard from again!