persephone wrote:...Diameter is really only dependent on strength needed and the specific line used.
With modern materials the breaking strength (or even the working loads) of very thin line are probably already adequate. For boats like the Daysailer, the load is really no longer a useful criterion in and of itself.
The weight of a line might matter, although, if it's not weight aloft, I wonder whether a pound or two in weight really make that much of a difference when the hull isn't exactly a lighweight to begin with.
For halliards, I might buy the argument that less weight aloft can be felt, because of the leverage arm, but sheets are usually led fairly low.
In light winds, very heavy sheets could create enough of a pull by weight alone, that it interferes with the set of the sail. How easy the sheet runs through its blocks would seem to matter as well.
They are also harder on the hands when sheeting in under load.
I like a sheet that I can grip comfortably. That's the reason why I have no problem using sheets that are ridiculousy oversized based purely on load considerations.