Bigger lines don't necessarily cost more. Lower quality rope needs to be bigger to have the same strength and stretch characteristics as smaller diameter higher tech rope.I used 7/16" for the sheets on my C&C 35. I'll never understand why some people (and owner's manuals) use big lines for their running rigging. Bigger lines cost more, they often don't run freely through the blocks and they weigh down your sails in light air. I was guided in my choice of line sizes by the cordage manufacturer's recommendations. IMO, they know their own products better than the writers of owner's manuals.
I have dealt with both extremes super strong small diameter lines on race boats that shred a pair of sailing gloves in a season, and big fat double braid thatis comfortable to grip but creates a lot of friction through blocks and fairleads.
The trick is to find a balance between the two extremes, and that really depends on the size and type of blocks and cleats your specific boat is rigged with.
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