I loathe stringing BJDs!!! I know some people don't mind, but it just makes me a bit more crazy than norm.
I think (not my strongest suit, lately) that you can simply measure the diameter of the rope to see what the size if, assuming you have a ruler or measuring tape handy, of course. I'm sure the size would definitely make a difference, but I personally have had to just make adjustments to get my dolls strung just right... Which reminds me that I have a guy that's too tight and needs some heavy adjusting... Nghhh....
Good luck with your new dragons!!!!!!
Post us some pictures when you get them strung up right!!