Wire-to-turnbuckle size

[benz]

Can anyone tell me where to find a table or a guide to determine what size of turnbuckle is suitable for a certain size of wire? If you look at a 1/4" turnbuckle, it is ridiculously small for 1/4" wire--it seems they always use several sizes up. And also, does anyone know why the 5/8" shackle I just bought has a 3/4" pin? I'm trying to get my chainplates drilled prior to installation, and rather than rely on measurements, I've simply bought some turnbuckles and shackles so I can make sure all the pieces will fit together with proper clearances before they're bedded and bolted on. But I keep finding that shackles stamped "5/8" have bigger pins than that, and that turnbuckles labelled "5/8" have far skinnier material around the eyes than that, and don't even get me started on propane fittings that say "3/8".
While I'm here, a guy at the hardware store told me the safe working load of the aforementioned 5/8 shackle was 4,000 lbs, and breaking strain was 8,000. Can something this big really be so weak? If anyone knows where a table of specs for shackles can be found, I'd be keen to know. By the way, I'm dealing with all galvanized hardware here.
Thanks in advance,
Ben