I'm not a Buccaneer sailer, but I've got smaller daysailers and have crewed on Flying Scots.
There's a tradeoff between line strength and size. With today's more advanced synthetics, strength is of no concern. So now we get into size.
The primary concern with line size is feel in the hands. Well, and weight, too. So, the larger the diameter of line, the nicer it feels on hands. However, particularly with headsails, the larger the diameter, the heavier the line, and the greater the possibility of the heavy line affecting jib shape in lighter winds.
There is no way I'd sail a boat with 3/16" jib sheets. That would kill my hands. On my 15 footer, I started with 5/16" for jib sheets, then went down to 1/4", though I don't think the smaller line was significant to jib shape in very light winds. I do use 3/8" for the mainsheet. Again, this is primarily a hand comfort issue, though the multipart purchase on a mainsheet obviates this concern somewhat. I could easily use 5/16" if I wanted. Generally, the jib sheets have a greater direct load, being as they are not multipart. (On a Flying Scot, they can be rigged straight or 2:1, as well as changing the main from 2:1 to 3:1)
For my 18.5' keel boat, I use 5/16" for jib sheets, 3/8" for mainsheet. The main is 3:1.
I also use higher tech lines than a polyester double braid such as New England Ropes Sta-Set or equivalent. For my little boat, I use FSE Robline Racing Sheet, which is a single braid blend of Dyneema and Nylon. For the bigger boat, I use New England Salsa, which is a single braid blend of Dyneema and Polyester. Both lines have a great feel on the hands for good grip, while being soft. The Racing Sheet tends to flatten in cam cleats (as one expects from a single braid) where the Salsa does not flatten. And the Racing Sheet is more expensive than the Salsa. The Salsa runs through blocks extremely well, so much so that it is preferred by Hobie 16 racers with their 8:1 mainsheet blocks. In my opinion, the Salsa is just plain ugly, with all kinds of flecks of color, I guess to make it look like a bowl of salsa. Oh, well, it's damn fine rope, so I suck it up and use the grey color salsa as the least objectionable
Salsa will also get a wooly look to it, with strands kinda sticking out. This bothers me to a certain extent, as I don't like my lines to look worn or furry, but again, in 2 seasons the wooly look hasn't got worse, and the line remains excellent, so it's something I just overlook. Salsa is reasonably easy to splice, directions are on New Englad's site, but my mainsheet splice wasn't as easy to do as I thought. The brummels aren't as tight as I want, and the eye over the becket didn't stay as small as I wanted. Still, it holds very well, so I've had no reason to try to redo the splice.
