Hi David,
all my experience of ship modelling has been wooden models, but here are a few comments which may be useful.
As has been said before, the rigging size depends on the type of ship. A fighting ship will have heavier rigging, to help it stand the punishment it will get. A merchant vessel will have lighter rope because it is easier to handle, spare rigging takes up less space in the hold, and it keeps the weight down at the higher level, so lowering the centre of gravity.
The shrouds, since they support the masts, will be similar to the other standing (i.e. fixed) rigging, while the ratlines will be much thinner. The standing rigging, which has to stay in position for many years, was almost always treated with tar to stop it rotting, so it will be black.
As Don said, err on the thinner size. I have found that if I use a thread that is correct for the scale, it always looks too thick. This is not helped by the fact that most threads are not totally smooth, so the 'fuzz' on them makes them look even larger. You can reduce this by rubbing the thread with beeswax, but then you can't glue them, and I find a small spot of glue on the knots make them much neater. Unless you are making a 'museum' piece, so the scale has to be perfect, use thinner.
I have just finished a 1:50 scale HMS Bounty, which is about 29 inches long (the actual ship was about 115 feet long), and I used 0.6mm dia. for the shrouds and 0.15mm dia for the ratlines. This is not much over half the correct scale size, but I think it looks OK.
As an example, HMS victory had shrouds which were about 2.3 inches dia, and ratlines which were about 0.5 inches dia. She was a fighting ship, about 220 feet long.
Bear in mind that if you use a reference book for rigging sizes, that in the 18th and 19th century all rope sizes were quoted in circumference not diameter. So the shrouds on the Victory were quoted as 7 inch rope. (The anchor rope was 24inch ! - still quite large at 7 inch diameter but not as big as it sounds).
Hope this helps. The main thing is to be patient. I found on the Bounty that the rigging (of which there was lots) took nearly half the total build time, but it cam make or break the final appearance. Also be careful about not making the rigging too tight (which can distort the model) or too slack (which makes it sag). It is sometimes better not to fully tighten the knots or cut off the excess until a whole section is finished.
Good luck,
BOB.